What is going on with my stepdaughter?

My stepdaughter, 15, stays with us every other weekend and every Wednesday. Her father, my husband worked away for years but she still came to stay with me and her sister, 2. Her father came back to work in the city about 4 months ago and now she hardly comes at all. She has cited school work as the reason. I know her mother is very influential over her and when she found out that some mutual friends of hers and my husband joined us for a BBQ, she gave them an ultimatum to choose her or us. It's been really since then I guess. I cannot talk to my husband about it as he either gets angry or doesn't give a ****. I miss her and I am sad that she has become so distant. It's hard to get a conversation going with her and she is vague and avoidant about all subjects. My daughter and I bought her birthday gifts. I thought she would take them home but she left them with us and I felt that they didn't matter to her. I am so hurt. I had thought of talking to her mum but I rather suspect she is behind this. I don't think talking to my stepdaughter will help as I think I will be met with a wall of silence. Am I afraid it's me? Possibly. She came this weekend and it was uncomfortable. It's like egg shells around the house. I felt so alone in my own home. To make matters worse my own mum left today after staying for a month. I live in another country. So I am sad. I'm sad because I can't tell my hubby how I feel (he has Aspergers) and I am sad that my stepdaughter has become a stranger. Crying................