Nobody knows what goes on behind closed doors.  There's nothing you can do about her decisions.  I understand from the child aspect, you are incensed.  Just try to take deep breaths and tell yourself, "What will be, will be".  I'm sorry for you and your family.

If I were your dad, I would take it all and leave his STBX in the gutter. Regardless of her being the mother of his adult children.

He needs to get a killer shark attorney, meet with each of the top 10 divorce attorneys in their area for a consult... to take the talent off of the table for use by his STBX.  Then destroy the STBX.

She did her PhD while they were married, there could be a good case for him to be entitled to a % of her income for the rest of her career/life plus spousal support, and half of her entire retirement accumulation (401K, IRAs, etc...), they house, cars, everything.  He needs to see just how much she wants to start hitting the geriatric dating market. Once she tells him, he needs to write off the marrige. Even if she changes her mind once she sees she may have to live the rest of her life on a TopRamen budget.

If either of my parents pulled this crap on the other, I would be full on team destroy the cheater. Of course I would still love the cheater, I would forgive the cheater, but I would be helping the other parent build the Karma bus and take an occassional turn at the wheel to repeatdly run over the cheater. Figuratively of course.

If your dad takes most of it, your 67yo mother may just find that her Cougar prospects are decidedly limited.

I hurt for your whole family. You, your sister, your dad... and even your morally and character deficient mother.

It is pretty obvious that there is not much grey in my world. There is right, and there is wrong.  Barring any extenuating information of course.

My XW had a dating problem the entire blessedly short time we were married (2.5 years). I have zero tolerance, zero understanding, zero empathy, and zero compassion for someone who would do this to a spouse apparently just to get their tingly bits serviced.  Ceteris Paribus.

As for what to say to your mother..... I'm so tempted to tell her that she's lost her mind and that I'll never forgive her for hurting my dad. Sounds good to me.

I don't think we can judge other people's marriages, even if they are our parents. And what appears to look certain way from outside, might not be that.
 

 I am really appalled by advice given to never forgive her, tell dad take everything from morally deficient mother (it doesn't work that way, you can't just demand everything in divorce because you are angry). That's just crazy making.

As much as it hurts you'd have to accept your parents' life decisions. And I am sure there is more to it than just her wanting to date. 

If you were unhappy in a relationship would you choose to stay and be miserable?  Do you assume because your mother is 67 she's beyond the age to find a happy relationship.  Obviously something isn't working between your parents and you really shouldn't judge them for being human.  

Keep in mind also that sometimes people in the early stages of dementia do crazy things like this. 

Perhaps she has felt that she HAD to stay married for all these years from duty but didn't feel in her place - that would explain her spending so much time at work. Perhaps she finally feels that she can live her life for her now that her children have all launched and her husband has retired ( and is possibly building a new life too)? Who knows what her motivation is? Dating doesn't necessarily mean she's looking for a new partner, just that she wants to experience things that she's never been able to.

You have only heard your sister's interpretation of what your mother wants, try to reserve judgement until you have actually talked with your mother.

You are understandably upset. They're your parents and you love them. But I agree with others who say you don't know what's been going on behind closed doors.

If you take sides and take your anger/upset out on your mother, it could damage - if not ruin - your relationship. Do you want to take that chance?

IMO, your best course of action would be to stay as hands off as possible. Don't take sides, don't try to destroy (OMG) anyone. Suggest counseling. Offer to be there, but not as a sounding board for who did what and why that person is the bad guy. You can be supportive and loving without getting in the middle. {{{HUGS}}}

My friend's mom did this, but she had already met someone else (apparently she reconnected with a high school sweetheart over FB) when she decided to divorce. As my friend told her when she shared the news, "I'm not surprised because I think you've been unhappily married my entire life, but this is the dumbest financial decision you've ever made." Come to find out, her father also has early signs of dementia, which my friend discovered when he went missing and his friends contacted her. He later got arrested (nothing major, likely resisting after a traffic violation) in some tiny town and she had to find a lawyer and get herself down there to help. All that is to say that there was a lot going on behind the scenes, but it's not unheard of for people to divorce in their later years.

Your sister should not have broken this to you. My parents should have divorced when we were in High School. She asked our opinion and we said "how could you do that to dad?" So she said she'd wait until we were out of HS (two years later). She died 5 months after the divorce. She loved our dad and he her. But there was no affection there. Not to be macabe, but life is short and we don't know how long we have. In their 60's or not, it is none of your buisness. Try to support them both.

No one  but the one making the decision can ever know all of the feelings, thoughts, or even the facts motivating the choice..

But when someone chooses to not be happy after 40+ year  of marriage, ostensibly because they want to date, the onus of responsibility is on the one makeing that choice. 

IMHO of course.

If it was one of my parents doing this to the other, my brother and I, and our families after 40+ years, I would be the adult SKid from hell.  We all have plenty of examples of that to draw from.   And.. I would make sure the whole fam-ed damnily had the facts of that parent's choices.

I would not wish this on anyone to go through. And I am sorry you, your sibs, and your dad are going to suffer because of your mother's choices.

I agree with most of the advice saying that you need to try to understand that it's easy to think you know what is happening from the outside.. but the reality could be different.  It does sound like your parent's relationship had it's challenges.. and in fact, was probably one sided.  It's also possible your mom is having some sort of "midlife" crisis.. wanting to find that spark of happiness before it's too late.  The reality is that we all know it can be ever elusive.. and many of the problems in our own relationships are totally of our own making.

I would listen to her with an open mind.  I would be mindful of clues as to any potential mental decline by her or your dad (given their ages).  I would also do my best to support both of your parents to the extent possible without getting overly involved in the relationship part.. but your dad and mom will both likely need to know that you support whatever decision that they, as adults, have made in their lives.

Give your mother an open ear, try not to judge, and love them both, but dont get into the murky waters of emotional involvement.

Theres a lot you dont know.

Any updates?