Where's his father in all this? Does his mother have a job? 

If you are living like he is your own, treat him as such. You can't help what the grandparents do but you can do something about what goes on in your house (unless you also live in grandparents home.) No gaming except during x hours. Disrespect gets all electronics taken away. I do feel that it's right to include him in family dinners and trips if everyone else goes, but your wife should contribute fairly, too. You can't just keep paying for him but having no control. Maybe family therapy will help you set boundaries gradually. It would suck to leave your bio child with all of them. Don't let him win. Stand up for yourself. 

I feel ya

I started to resent my former skids because their Disneyland dad and trifling lazy conniving breeder raised not one but two no-home-training spawns who were loud, messy, demanding, disrespectful, relationship destroying, codependent domestic t3rrorists

My stance is if you expect me to use one iota of my resources on providing for/helping your demon seeds then you best be prepared for the fact I will discipline and hold them accountable if you fail to do so. 
 

Disneyparents don't want you to say anything to their kids when they need to be checked but they have no problem using your hard earned money to subsidize their child support/using your effort n ernergy raise their spawns

If a bio parent has a lackadaisical parenting style and expects me to provide for their kids but doesn't allow me to lead in household parenting when it's necessary to do so then I'm going to peace out of that living situation and/or relationship 

*I saw the dysfunction before I got truly locked in (through marriage and bio kid together) .... had several conversations which he refused to listen / change for the better .... so it left me no choice but to leave and let him go back to single parenting his clingy no-home-training kids .... they start to understand how hard they make things when they have to do it all themselves 

**im not saying you should leave your spouse but you do have to have a conversation with her and exert your boundaries in the home YOU pay bills in....otherwise your spouse is skating on thin ice with respect to maintaining the marriage 

***her son either follow your rules or he can pack up and go live with grandparents...and if your spouse dare challenge you then she might end up living with her son at grandparents house too

She is letting this go on.  She can tell her family to stop buying SS everything under the sun.  But I guest BM is guilty of not giving her son his Gapoy Family.  This is a you and SO Problem.. That must be solved first. Unfortunately you have a kid with BM giving her the upper hand.  She knows you want your kid to have a happy family.

SS is 14 a few years he will be going to university 

It is your wife that you should hate over this. Though the toxic failed family spawn certainly is old enough to get a foot up his ass for his shit behavior as well.

Time to stop supporting SS.   Make sure that you are pointing out to DW and your puke worthy ILs the inequity in how they treat your DD.  When they spend on him, demand that same amount for your DD then invest it in her education fund.

That mommy favors her shit spawn of failed family genetics over your/her DD says more than anyone needs to know about your idiot wife.

I would stay and make both DW's and her failed family spawn suffer the full facts of their bullshit.  Cut off DW from any of your money. She pays 62.5% of the bills in your marriage. Her full share, the full share of her failed family progeny, and half of your DD's support needs.  No need to leave. Stay, and make your DD and yourself your focus while letting DW know that she is welcome to be a quality part of your life and your DD's life if she womans up and becomes an actual quality partner for you and mother for your shared DD.

When the 4 of you go to dinner tell the server to split the check. She can pay 62.5%, you pay 37.5%.

If she makes a big deal out of it, walk off after paying your share and take your DD for a short walk until your DW gets her shit together and she and SS join you.

Lather, rinse, repeat.

Her shit parenting is disrespectful to you and to your shared DD.  Do not tolerate that. 

K.I.S.S. Keep it stupid simple. She either learns or she and SS suffer.  Insulate yourself and your DD from them as much as possible while being fully present for your DD.

DW should demand that her DS respect you. And what yiu are doing.  But instead, she's letting SS disrespect you.  You have PTSD from all of this when the water bill becomes a point of contention.  All kids should be treated the same.  I get it that they have different interests, want things. But I always do the money thing. If a kid gets a $100 gift the other kid may get 10 $10 gifts.