D.B.
I'm a stepmother and I get what you're saying.
The problem is: your husband's family hasn't dealt with the divorce. They are treating your badly, and your husband puts up with it. Stop there. That's the main issue. Your husband has kept his daughter close by not standing up for you, and she sees that it's working. So she continues to control and manipulate by engaging in this behavior. If he includes you, she pulls away and THAT is what is causing them to grow apart.
The family could have stayed on cordial terms with the ex-wife without excluding you. But they disrespect your husband by refusing to accept his choice of a wife. And your husband puts up with it by continuing to take you into family events where you are mistreated and not welcomed. He needs to put his foot down.
Your SD came into your life at a tough age - teen years. That's when she's figuring out the kind of woman she wants to be and the kind of man she wants to be with. She's learned, with plenty of encouragement, that she can pit 2 women against each other (her mother vs. you), that she can manipulate a man (her father), and that she can join up with a group of people (the relatives) to bully another woman (you). Great skills for adulthood.
There's nothing wrong with putting up a picture of her parents on her Facebook page. That's if the rest of this nonsense weren't going on. But she's done this as a passive-aggressive way to say "Screw you" to his father's wife. So it shouldn't bother you, but her reason for doing it should.
You and your husband should get some couples' counseling so he can figure out how to have a relationship with his adult daughter, make up for some of the mistakes of the past, and how to value his wife. Teaching his daughter how to be valued by a man is a great side effect of this treatment. Your stepdaughter has some behavioral problems, but she has been helped along this negative path by a whole lot of people. So the only people you and your husband can control are yourselves, and the way you react to treatment by others. Start there.