But when I read the the MIL pays for the kids phone, yep, she has the right to control it.  

Wow, OP, don't take this the wrong way, but you are waaaay overinvolved in all of this skid drama.  Just disengage and let MIL do what ever she's going to do. The skids are adults, and if they move in with GMA to avoid rent, so be it.  Why tell her about the drinking and women in his house, etc? Who care if they hang out with DH's ex? Who cares who SD is or isn't dating?

Just disengage and focus on your own life, your marriage, your own kids, and stop being involved in this nonsense. 

You are way tooooo involved with these kids.  I had forgotten you were the one with thethe Father’s Day dinner fiasco because you wanted girlfriend and boyfriend cell numbers.   Take a giant step back and disengage.  It just might bring you some peace. 

"I knew that if I want change, I need to go to her because she is the matriarch of the family."

It sounds like you desperately want someone to hear "your side".  I get that completely!  Unfortunately, I have lost my sh@! a couple of times and unloaded all that I deem to be unhealthy in her family.  That didn't work out so well and I had to work pretty hard to rebuild basic civility between us after that for the sake of my relationship with MR. ED.  

What I've come to learn dealing with enabling GM:

• She's always been an enabler, knows she's an enabler and will continue to be an enabler.
• SO is responsible for setting that boundary with GM, so my issue is with him.
• SO's family doesn't communicate well if at all, but they actually prefer it that way.  The less they know the less guilty they feel about enabling dysfunction.  There's no changing an older family matriarch on that one.
• She will never come to my defense over her DS or SKIDS.  To validate "my side" fully would mean she would have to acknowledge the dysfunction in her own offspring and in her raising of children.  Not going to happen.
• Matriarchs don't take kindly to other women telling them how they screwed up raising their kids and grandkids.
• She will probably turn that right back around on you when it comes to "sticking her nose in everyone's business as if she know's best."  As far as she's concerned, her DS and his kids are HER business, not yours.  That was a hard one for me to wrap my brain around because it's my relationship, my house, my finances, my sanity.  But...I've heard the other side of that thrown right back in my face.

If MIL wants to enable SS, I'd try to let that one go.  At least he is out of your hair, out of your house, and not being enabled by you!  For what its worth...I hear you! <3

I’m pretty much in her same boat as you. I feel your frustration. I have disengaged. It’s doesnt stop them bitching behind my back, but I can’t bear the fake niceness in person then mil sil and step d all go bitching afterwards. I now only have contact with my brother in laws wife and their son. I am learning to ignore their opinions of me, - but I don’t like being blamed for things I haven’t done, or things that have been twisted. 

I hope things improve for you. 

My mother is like this, if I sent her a text like that she would go radio silent, until she wanted to put me on blast to everybody, then she would come out guns blazing. I have learned with my mom once she thinks she's right, it doesn't matter how valid your arguments or feelings, she won't listen or care. So I have learned to disengage from her, it makes things so much easier. I still love my mom and interact with her, but I refuse to engage in any arguments or if she thinks she's right about something I just smile and nod and move on.

The only thing to keep this text from being a complete waste of your time is that perhaps you got some things off your chest.

Now, please make it your swan song and go No Contact with this woman who happens to be related to your H. She isn't a friend to your marriage, has been stomping boundaries for decades, and is a huge contributor to the dysfunction that exists in your H's baggage.

Look at it this way: right now, you're winning. SS is out of you house and wallet, and you and your H can start to move forward in a healthier way. But it starts with YOU. You've simply got to start controlling your thoughts and actions. Either you're participating in the drama or you're not, and at this point you are overly involved in all of it.

Just drop the rope with ALL of your H's people. Stop interfering, stop overfunctioning, stop, stop, STOP because you are only causing more damage. Get a therapist or vent here, but find a way to release all of that frustration and resentment that rightfully should be pointed at your H.

Came back to say, try just being your H's girlfriend for a while. Let your focus begin and end with just the two of you. His kids, his mother - all of that is outside the tight circle of just you two and is up to him. Pretend you don't know any of them, so of course their issues are none of your business.

Hopefully this technique will help you jumpstart your disengagement.

 cult is what you married into.  So sorry.  I have been blamed by so many people for decisions DH made that I disagreed with, but nothing you can do about it I guess.   Best of luck to you!  Sounds like you are better off, OUT of this crazy family.

"I reminded her that I am the one buying the presents, mailing the packages and cards, planning the trips to go visit and the dinner with their dad."

Yeah....I feel this burn. I just wrote about an abysmal graduation experience where we were publically disrespected and treated terribly. What did I do? Released balloons into the sky. After that experience I dialed WAY down. I don't buy gifts whatsoever. I used to buy christmas gifts, birthday gifts, fund vacations twice a year- heck NO! Not appreciated and SKIDs never recognized my efforts. A fun RECENT story you can relate to - is after copious amounts of wine in the downtown district I realized it was getting close to one of adult SS birthdays...in the window I saw an amazing shirt that he would LOVE. I run into the store spend a good amount of money on this cute polo and ship it off. At first, adult SS thinks dad has given him the shirt- he GLOATS about how great it is! Then DH corrects him and says I've picked it out. I get an obligatory & grumbly thank you along with a complaint that it is now all of sudden too small and that I failed in picking the right size. Lesson learned AGAIN. 

Don't do anything extra- truly. I am getting better at not doing that and spending that hard earned money on myself. Also if you really feel like being generous (which I do from time to time) go find a wonderful person in your life to put that funding towards. Yesterday I went the extra mile for a barista whom is just a joy everytime I see her. I took money and gifts and gave it to her. Guess what? She was grateful ! No complaints only a huge hug. 

I feel this...my MIL is very similar. I know I'm not going to change her. DH has talked to her and shared specifics about what he needs from her, including that he needs an apology from her for some really egregious things she did. She refused and told him he just needs to "get over it'. We now just let her spend all her time meddling with SSs and keep her at arm's length. She is not going to change, but we have put up walls around our house. I have said a few things to her, but have not let her know what I truly think of her and her dysfunctional, mean, and unhelpful behavior, because I know that if I speak up, I become the bad guy. Instead, I'm just biding my time until we can move away from her. I know that she says things about me that are not nice to SSs and likely to the rest of DH's family and it still bothers me, but I have to not get wrapped up in it for my own sanity. I have a counselor...who I started seeing to deal with HCBM and the dysfunction of DH's family. I once said to the counselor that it hurt me that DH's family did not see me as part of the family. The counselor pointed out to me that it didn't seem like they saw DH as part of the family (they don't...or rather they treat him like his divorce revoked any rights and privileges he has to manage his own life without their meddling), so I shouldn't take it personally. 

People like this will not change....they always have a target. Unfortunately, you are now MIL's target. The best thing you can do is distance yourself from her and force her to find a new target.