Here's me experience. I am a step parent of child with behavioral issues. He couldn't go to a regular school because the issues were so bad. I also had a daughter with my step sons mom.

To be blunt, there was nothing but regret for me in this situation. I still had to maintain the rules of the house even though my stepson fought me the entire way. My experience is that Moms tend to be over protective when a child has a mental disability so in my case, every time I attempted to discipline my stepson, I had to also argue with his mother about it. Regardless, I stood my ground because there was a little girl in the house and I had to make sure she was safe and cared for. Every day was a battle and life became unbearable when he got into his teens. His mother and I were not on the same page. I would have left the relationship in a second if it wasn't for my daughter. I live in an area where the Mom always gets custody regardless of the facts so I stayed in the relationship to make sure my daughter was safe.

My step son is in his twenties now and still lives at home. His mom is still constantly throwing all her money at his mistakes and he isn't learning any life lessons. He works but he calls in a lot. He hangs out with a bunch of unemployed losers who also are living off of a relative. He smokes a lot of weed but can never seem to afford to contribute anything to the house. He's been in jail several times and is probably headed there again. I have been trying to get him out of the house but his mother is always standing in the way because she can't bear to watch him go.

The whole experience has been hell for everybody involved. My advice to you is that if you have a way out then get out. It's just not worth it.