A: Stepparenting is challenging because of the circumstances that led to divided families, and kids' reactions to their new reality can vary greatly. Your stepdaughter is clearly struggling, and her struggle is having effects on your family.
I believe a two-step process is in order here, and the girls' father needs to be central to both steps. The first step: sit down for a "family meeting" that includes the girls, you, and their father. Your husband should lead the meeting because his girls are more likely to be responsive to what he says than to what you say at the start. He should say the meeting is not about blame or punishment but about love and family. Discuss the tension and the behavior issues in a calm, non-threatening way. Hear what the girls are concerned about and (gently) share your concerns. If this begins to decompress the situation, have more regular meetings until you sense everyone is on the right path towards healing. Chocolate chip cookies and cocoa can help a lot at family meetings like this.
If step one doesn't work, here's step two: Arrange for a family therapy session with a trained therapist who will mediate the meeting. Show the girls that you care about them and that you are committed to making this new family work. Hopefully, in time, they'll appreciate your efforts and contribute to the process as well.