Toxic in-laws have no place in your life. Distant but polite in the future. See them when you are obligated to (Christmas etc.) and any unsolicited advice is replied to with "hmmm, something to think about", then do whatever the fk you want.

Or, you could stop interacting with them entirely. The thing is, though, to not rely on them for anything. That way, you can walk away and not be affected. If they are doing babysitting or paying for anything or housing you, they can use that for control. 

Grandparents LOVE to meddle because they think they know everything. And 9 times out of 10 these toxic MILs enable their sons bad behaviors (which is why we have so many Disneyland dads, men who don't clean up after themselves, insert other unsavory behaviors here)

I'd flat out tell her "Look Betty, YOU had x number of kids and raised them how you saw fit. Now, I am fully grown, we pay the bills around this house, these are MY kids and I will raise them how I see fit" 

And you sound like a great parent just from the fact of adopting the step kid and making him like your bio kid even legally....that's a huge committment and you took on the challenge willingly and lovingly. You adopted him and he is YOURS just as if he is a bio kid.

That lady needs to back up a bit and know her place. If she's not paying major bills for those kids and As long as those kids aren't being abused or neglected then she needs to hold her opinions. Good thing your husband backs you up. 

Step-parents are always seen as 'evil' (especially if we rightfully demand change from the dysfunction that was okay with their low quality exes or enabling mothers). Yet, we are helping these exes raise their kids, diverting some of our resources from ourselves/our needs and wants and funneling it to those kids, some of us providing more emotional/financial/physical support than their own bio parents, etc. These bio parents expect us to invest in kids we have no rights to (unless we adopt them). So IMO unless they're welcoming/cooperative, the bio parents/family are the 'evil' ones. 

I think your situation is complicated by the fact that prior to you - your DH was letting his parents and sister pretty much raise SS. Even though you have all worked through that - I would guess there are still times that they resent the fact that things changed so much after you came into the picture and feel like they should still have some say in how your son is parented.

That said, it has been 4 years since you adopted him and they need to get over it and stay in the grandparent role. They should never have said those things in front of your son. I understand why you lost your cool.

Is DH doing what he needs to do to deal with recovery? Are you getting support from somewhere like ALANON? Hang in there, it sounds like you have really been through it.

are usually crap.

I too adopted my Skid. At his request when SS-30 was 22. His mom and I met when he was 15mos old and married the week before he turned 2yo.  I raised him as my own.  We discussed adoption when he was in his tweens and early teens.  He gave me clarity.  I was his dad with or without papers.  He did not want to inject that additional drama into his SpermClan visitations.

As his mom, you are fully in your purview to put toxic people in their place. Including your MIL. That your DH has not already put his mom in her place, is alarming to me.

I hope your son's final few years as a minor improve significantly.  

Take care of you.

People try to run others people's life's.  They should concentrate on running there own life.   Back to disengaging from MIL and all other unhealthy people. 
You parent each kid differently,  every person ( your kids) are different people and must be parent differently.  Trying to get to the same result.  Functional Adults, who can live there own life. With out help from the world.  Have jobs, have a place to live buy themselves.  Have good relationships.