Dear Stepdaddding,
I have two biological children with my current husband and three 
children (two daughters, 18 & 16, and a son who’s 10). Over the past few years he had a falling out with the oldest. The younger of my two daughters felt that speaking to her stepdad would be betraying her sister. The oldest moved in with her bio-dad. The 16 year old is resentful, upset and despondent.
Me: “Let’s go to therapy and try to work this out.”
Him: “No, I’m done with her she just uses people to buy her stage, she is not a good person.”
Daughter: “Why should I have to be the one making the effort he’s the adult.
Now my husband wants to plan a family vacation but the 16 year old is NOT invited, even if I pay for her. Yes the child is stubborn, doesn’t know when to stop and lacks a common sense and empathy. None the less, she is a young girl that needs discipline, acceptance and forgiveness. What do I do? Please help me, help my family. – IVONNE
Ivonne,
Your daughter’s an average self-focused teen. She’s spoiled and has a sense of entitlement but she’s right about your husband. He’s an adult and a parent. It’s his responsibility to lead. His passive-aggressive way of dealing with her is surely a pattern you’ve seen with him. But it has to end. It’s destructive to the relationship he has with all the kids and destructive to the family.
Your suggestion for therapy is spot-on. But go beyond a session with just the two of you. To change the situation you all need meet with a marriage and family therapist. The damage is to the entire family group. 
You need to work together as a family if you want to stop the destruction and hurt feelings. This will be the best way to start the healing process. Make the appointment for all of you, your husband, yourself and your two oldest daughters. Tell him he has to go. Do whatever you need to do to get him there.
His behavior creates a negative environment in your family and sets a standard for how the kids will be treated in the future if they misbehave. Your family is on thin ice. Take immediate steps to stabilize the family. Fix this now. Ignoring the situation will only allow it to worsen. If you don’t find a way to repair the family group your marriage will continue to deteriorate and ultimately end.
Good luck. You can do this.