Family counseling, stat. We are also a blended family and after my step-daughter moved in with us a few years ago at age 13, we did counseling for the whole family. I wish we had done it much sooner, when we first got married as my husband and oldest son have a very poor relationship.
One of the great things that the counselor did was call everyone out on their BS, privately (well except the little boys). He told my oldest son that if he didn't like being blamed for everything and being harassed and punished all the time to stop acting like a tool and that his behavior had the power to set the dynamic for the whole house. He told my step-daughter that she wasn't some special little island who could just stay in her room and ignore everyone and pretend we weren't there - this was a family and like it or not, she's a part of it so get on board and participate and stop making everyone else try to read your mind. I was told to not coddle my oldest son. My husband was told a crapload of stuff about how he interacts with everyone.
There were lots of action items to go around - rather than feeling blamed, it was framed in a way where the counselor emphasized that we ALL contributed to the dysfunctional dynamic, we weren't victims, and we ALL had the power to effect change. It was very empowering and helped us a lot.
You sound very frustrated, and I would be too in your shoes, but please have hope. You and your husband and your kids can create a common vision of what you would all like home life to be, and you can all work at that goal, together. You've only got 5 more years with your older step-daughter at "home" with you - don't think of that in terms of "I only have to get through 5 more years of this" but more in terms of "I only have 5 more short years to help her learn and grow into a wonderful young woman who is ready to go out into the world after high school." Take some time to picture with your husband what successfully raising her will look like - what kind of person will she be? What will your relationship be like? What shared experiences will you have? What will she treasure about you, and you about her?
Then look at the gap between here and there and start to map out a plan to heal the relationship (for you), put appropriate boundaries in place (for him) and set reasonable expectations for visitation and spending time together. Older kids will naturally want to spend less time with parents because of their social lives - so make sure that you can accommodate her social life within reason, especially if her visits with you are normally on weekends. And your husband has to show that he values extended family - yours and his - by spending time with both sides and not making it optional for any of the kids.
Anyway...step-parents can be one of the most influential people in a child's life. Make your influence a positive one by working with your husband and a counselor to help correct the dynamic. It can be done - stay positive and good luck!