I'm going to take a slightly different tact. I pretty much agree with what everyone has said so far, however there's one thing everyone should keep in mind: given your D's age it would be best, for the both of you, if you let her manage her relationship with her father. She can and must begin to learn how to handle her relationship with her other parent and to not come to depend on you to step in for her. (It would be a little different if she were much younger.) Your D can certainly talk to you about such things and you can be her sounding board and source of support, but to preserve your own boundaries as well as to help your DD develop her own healthy boundaries, her father is her own responsibility now.
Having said that, I don't like your exH telling you what you can or cannot say to your D about his family -- that is a case of his own "offsides" infraction. Your R with your D is your own, not his, and you have every right and responsibility to help guide her, even if it is in areas where he is concerned. (The flip-side is also true, however, so keep that in mind.)
I also don't like the fact they expect her to automatically be responsible for her step-family's child. That is over the line and highly unfair to your DD.
I feel bad for her that she is being forced to have to spend time with a step-family that tends to ignore her and take advantage of her. That's just unfair. Her father is being an insensitive jerk and is digging a hole for himself with your D that he will have a hard time getting out of if he persists.
But again, all of these are issues your DD has to confront herself. She needs to talk to her father about these things. If he fails to listen, that will end up costing him over time in his R with his D.
Your role, HM, is to love and support your D through these things and to help guide her through such difficulties, but not to respond to that instinct all we parents have to come to their rescue and defend our child against their other parent. I know, 'cause that's my knee-jerk reaction too -- if it were me, I'd want to go punch some sense into my ex's head too. But the right thing to do is to counsel our children and help them to learn to manage their own R with that other parent, and refrain from getting directly involved. This will help your child learn to develop her own healthy boundaries.
