That's where you are. Not officially a spouse. Yet. I think you need to see how your SO engages with her kids and how she treats your very valid opinions about her kids. Red flag items usually don't go away or get much better. Maybe I'm just being pessimistic. 

My DH86 is the male version of your wife.  We (I) finally got my lazy, manipulative lying, thieving SD62 out by arranging, with DH, to subsidize her shelter expense elsewhere ( she's on disability).  We separated finances and her expenses come from his income.

Like your wife, my DH cannot stand to hear a word against her although I, our other 4 kids, her own 3 kids and his own experience tell the truth.  We've rescued her from numerous jams over the years and bailed her out of jail twice.  Of course, none of this is her fault, right?

I don't talk to her or about her.  DH says she has always been there for him  (in my experience, sometimes) so he feels a loyalty toward her, especially since no one else is in her corner.  Like your wife, he's very defensive.   At his age, 86, I doubt he will ever change although it's costing him other family relationships.

I hate to tell you but I doubt your wife will change, either.  The "boy" doesn't sound like a bad person, he doesn't sound evil like some we hear about on here.  You probably need to decide whether you can live with the situation.  You sound like a kind, well-meaning person.  Just know that whatever you do or however much you do for him, he doesn't care for you and wishes you would vanish.

"Mental health is an ongoing dedication to reality at all costs".

He has been using both of you. I would withdraw from doing anything for this guy. No advice. No phone calls made for him. No suggestions. No vegetarian groceries purchased. No cleaning up after him. If SO asks why x room is a mess, shrug your shoulders and say pookie boy had been in there. 
(Does the other son say anything to their mom about pookie boy?)

If this situation is weighing on you on a daily basis and you see no change coming in the near future, you might want to talk with SO and perhaps discuss you living elsewhere. I can't bear to be around my mid-30s SS (the little prince) longer than 24 hours for similar reasons. . . it would literally drive me crazy to live with that.  

I am now at the point of telling his mother, she will end up alone with a parasite as a son, and the moment she is not useful, he will leave her and she will have sacrificed everything for him

My sister in law as well as a psychologist said this to my husband many years ago when he was a kid-puppet and allowing mayhem into our lives. It sort of sobered him up to reality. Might be worth saying! 

I am sorry you are experiencing this.  There is lots of enabling here in this situation by the mom. She would need to see the need to cut off the support of her grown son, in order for real change to begin. She is doing him a great disservice, allowing him to be "lame" to never function in society without her. It seems like she is a codependent, having the "need to be needed" to have a sense of security and superiority, to help hide her fears of abandonment.

Change needs to happen STAT if there be any hope for anyone involved. He feels entitled because his mother always catered to him.  He doesn't see the need to rise up beyond his lazy comfort zone because he knows he will always have his mom to cushion him. He isn't giving anyone any problems per say, because he is getting everything he wants and needs. Why would he complain? Now, cut-off his supply and watch the tantrums of a man unfold. It will get ugly. 

For 4 years my SD lived with us (we're married). She was told to leave back in January, to go back and live with her mother. She is 20 years old and did similar things your SS does. She was told to leave by DH because of her disrespect, defiance, dismissiveness, neglect, and the risk that came with that. 

She would put dirty dishes and containers of food in her closet, leave opened bags of food on the floor next to the bed, a crap ton of empty water bottles strewn all over the room (no exaggerating), left her heating blanket on HIGH on her bed with 3 blankets on top of it, regardless of how many respectful conversations we had with her to tell her to stop. We implemented a consequence of me doing random room checks at my will to make sure she wasn't doing these things. This was made very clear to her. She had the audacity to do it twice more! The time before the last, her father told her of she could not respect us and our home, if she could not appreciate what we have provided for her, she would have to leave. Well, that fell on deaf ears, and in January after a random check, I found the dishes, opened bags of food, water bottles, and heating blanket on high. It's like a big F-U to us, that she was going to do what she wanted, when she wanted. She played dumb when reminded of the "room checks." When her father tried to calmly talk to her, and tell her to bring down the food, dishes, and heating blanket, she defiantly told him, "No!" After the 3rd time, he told her to pack her things and leave. It wasn't my idea at all, but I am super glad it happened. No more cleaning up after an adult, spills, crumbs, hair in the drain, tons of rotting food in the fridge, the nasty attitude. No more, "Daddy, can I have $100 for a pair of sneakers?" "Daddy, can you pay this semester's tuition for me?" 

My husband and I are one = united. Thank GOD for that, because I could not take it anymore. 

See if you can have a calm, respectful conversation with your girlfriend, alone. Go out somewhere and talk and see what she says. I hope either way, you escape that drama and stress of this situation. It will not improve but only get worse without trying to communicate with her. 

He didn't become a spoiled, catered-to 'child' overnight. It sounds like she's enabling him and there's a long history of it. I'm so sorry to hear about the molestation; that's so horrible! 

That being said, I think you need to take a step back and decide how much of this you're willing to take. Again, this codependent relationsip between this adult man child and his mother isn't going to suddenly correct itself.  She doesn't want to hear what you're opinions are on the matter so nothing is going to get 'fixed'. 

Do you want to live the remaining years of your life in this toxic sandwich between the both of them.  

No one but YOU can decide if it's time to walk away now or not. However, put yourself first and love yourself enough to make some tough decisions.

Best of luck. 

Has your SO considered what would happen to SS26 if she could no longer work and take care of him?  Or if she was no longer around what would happen to SS26?  Is she putting enough away for retirement to take care of herself and SS26?  Surely, she has a timeline in mind of “when she will stop supporting” SS26?  If you were to leave the situation, would it affect her financially so that she could no longer support SS26 in this manner?   Your SO is an enabler and that in itself is a tricky issue to deal with.  You can still be amazing together and live apart.  I wouldn’t want to be raising somebodies grown man and supporting them on my pension that I worked so hard for.   

I agree with everything the posters are saying.  As I wrote above, I'm living with a similar situation.  I've come to realize that aside from pity and co-dependence with SD, my DH86 has some other feelings promoting his actions.

She makes him feel needed.  He feels like he's "giving back" for her support in the past.  As he ages, taking care of her puts him back in his active father role.  He's always bern a good father and feels this is an extension of that role.

I'm not sure what your SO'S motivation is but these feelings are extremely powerful.  Not an excuse, just trying to understand, like you.

"...his mother cannot see what I can"

This is the fundamental issue. And it's not going to change. Ever.

You are not being paranoid - you are facing the chilling and sobering reality that you are NOT first in your wife's eyes. That is why you are feeling like you are being used. Because you are.  

You are filling a certain missing niche in your wife's life (and in her lazy son's).  I am sure she enjoys what you do for her/the household - like the fixing, cleaning, shopping etc.  That means you are her house manager not her equal partner.  She doesn't want your advice on her failure-to-launch 26 year old adult son because that's not your place. You are essentially an employee. 

The choice is simple: If you stay in this relationship, you are subjigating yourself to being in a subserviant role in the household.  The adult son won't be leaving any time soon because there is no reason for him to. Even if he does, expect that he will boomerang back within no time at all. That pattern is established and won't change. And yes, it is going to be a life-long pattern. Your wife deep-down knows this and accepts it. She is willing to defend, support and sacrifice for her beloved son as long as she lives. She will not, however, do the same for you. 

In her eyes, the only person who seems to have a problem with this arrangement is you. I would make it easy for her -- I'd move out.

You are entering the supposed Golden Years of your life, where you have earned the right to enjoy retirement and relax. It's a time to reflect on your life and focus on what's a priority.  The sands are sifting down the hourglass and don't stop.

Not even for selfish spouses and their spoiled offspring.  Which your wife will learn the hard way. 

When you moved in.  There dysfunctional relationships was going on before you,  You, can not move in and change this dysfunction.  SS at 26 must go, SO is going to have to pick SS or you.  You most likely will lose. Be prepared to move out.  You can't change 26 years of dysfunction..   I don't blame you,  two adult men can't live together, SS will not be willing to give up his control.

Stop throwing good resources after bad. This no little different than any business or organization decision you make in your professional life.

Do not let the emotion of the situation and the sunk costs of your time and emotion cloud your clarity on the dicision you have to make.  Throwing more of your time and emotion into this shit show of a failed family is not a good investment and you know it.

Learn, take the lesson of this tragically failed partner, mother, woman, and her failed family progeny forward as you re-engage in living your best life.

Take care of you.

Like you, I have been in the corporate world for 35 years with nearly 20 of those years at the director level.   While family relationships are far more important than business ever is, organizational performance optimization strategies apply just as effectively in the family/relationship organization as they do in a professional organization.  At some point the accumulated experiences/data in a relationship give clarity that the sunk cost of time and emotion has failed and it is clear that it is time to cut and run from the forlorn hope of a lost cause.

I learned my version of this lesson via a blessedly short marriage to my serially adulterous XW. Interestingly, as you have sunk 2.5 years into your current tragic relationship, that marriage lasted 2.5 years from wedding to final divorce hearing.  I also wasted a year and half of dating and an engagement. I escaped without sullying my gene pool with her and before my federal felon convict inmate of an XMIL was prosecuted and the entire family saddled with a $multimillion civil judgement against them having to repay a portion of the embezzled $millions my XMIL scalped from her long term employer. 

I learned, I sought therapy and reconnected with the man I liked being. I invested in mystelf, sold my company to my business partners, redirected my career, and found an incredible woman to build a life with.

Gain that clarity, move on, and invest in living your best life taking the lessons of this travesty of a failed family with you to make sure you do not repeat the failure in the future.

Good luck. Enjoy your life.

You will NOT be one of the 2 out of 10 who will be successful. 

Even if you do it will be temporary. You are hoping to change a fundamental dysfunction in a family that is not yours and was a landscape that was created long before you arrived.  It was created by your SO, so the fact is she MADE it this way and trying to get her to change it is futile. It will never work. 

You are 58 years old and seemingly (based on your posts!) intelligent and reasonable, as well as reflective and caring. Those two qualities alone make you a hot commodity in the dating market.  You need to end this relationship and move on.

If it is too hard to snip all the strings at once, then suggest it will be a temporary situation and that you need to move out and be on your own "for awhile."  Find your own place, set up your own circle of support/friends and slowly disconnect. You don't need to make any big pronouncement that you are breaking up, splitting, etc. It can be done gracefully and without drama - you can say that it is clear your SO and her son need time to figure things out, and you want them to do that without any concerns about effects on you. 

Last but not least, I do think the fact your SO refused to take rent from you is/was a controlling measure. This way, she makes it very easy to tell you to get out if you don't like the way she runs her household.  She has manipulated you to contribute in other non-tangible ways such as houshold tasks, food, repairs, etc. but none of those give you the kind of agency/rights as a cancelled rent check would have.

I wish you the best of luck. Get out there and enjoy this chapter of life and don't even leave a bookmark at the past few years - they aren't worth a re-read.