I've been in a step-child situation with my ex-husband and first off, it can be EXTREMELY stressful even in the best circumstances, and horribly so if what you're saying about the other family is true. Ours was not an ideal situation as far as our relationship with the child's mother. However, he was also 13 when he came to live with us and it was difficult to say the least. For me. For him. For everyone.
The ONLY WAY we could get through (and we didn't divorce because of my stepson ~ it was long after we raised him) was for my ex and I to be on the same page about discipline, etc with the child. Period. We had to be a united front and if I were doing something wrong or something that he wasn't comfortable with, it was NEVER discussed in front of the child. He backed me up and we discussed it later. I'm not saying I was an angel either. I had never been a parent and I honestly didn't know what the heck I was doing sometimes....it was VERY hard but I did my best. And I tried as hard as I could to listen to and respect his thoughts and feelings regarding his child.
There seems to be a lot of different issues in your relationship but I gotta say that I think the issue with the child is probably something that creates huge amounts of tension, hidden resentment and unbelievable stress, which makes everything else a thousand times worse. And even if the child is a complete brat, not many parents are going to be receptive to being told their kid is rotten so if you're approaching it that way, you're hitting a brick wall and you're never gonna get through it.
We all have limits and boundaries and you can't have a relationship without knowing and understanding what those are...your own and your partner's. Sometimes, people can't change or they don't want to change and without professional help, they can't hear you. Or, perhaps, they only can hear themselves.
It sounds like a very disrespectful situation and I'm sorry for that. You can't be a doormat forever...everyone has their breaking point. I've also been in another relationship where I was constantly belittled and disrepected...after awhile, I started to just about believe the things he was saying about me and that scared the crap out of me so I left. It was hard but it was the best thing that I could have done for myself.
Now, I know what a good marriage is and I didn't settle for anything less this time around. You can be happy again!