Thank you for your email. It is not easy when you sense your child is in pain and you don't know how to help her.
Your daughter is at a very vulnerable age having to deal with changing hormones, changes at school, and now also changes in her home environment. It is important to understand that anger is pain and that she needs time, patience and support to mourn the loss of her family as she knew it and adjust to her new family circumstances. She may be blaming herself for the divorce and feeling she has no control over her life at the moment. She might also not like the idea of how the divorce makes her feel different when she is at an age where it is very important to fit in.
Read my articles: "Children & divorce" as well as " Meeting the needs of children in divorce" on my website. They will give you more information about what your daughter may be feeling and how to help her.
Do not let the fact that she won't speak to you about her feelings stop you from talking to her. The car is a good place to tell her how sorry you are that you and her dad no longer love each other and that although you tried hard you felt it would be easier to live apart. This does not mean that she is to blame or that you or her dad love her less. In fact tell her that you both love her a lot and hate hurting her this way and will never leave her and would like to know, when she is ready, how you can make the transition easier for her. Give her permission to contact her dad whenever she wants to and that she doesn't have to feel like she is betraying you if she does.
Continue talking to her in this way reassuring her without making a big fuss about the fact that she won't answer. Also offer her the opportunity to chat to someone she feels comfortable to talk to about the divorce even if it isn't you, or suggest other ways of expressing her feelings rather than taking them out on her sister, through writing in a journal, painting, exercising and so on. If she does write in a journal it is confidential and you should only read her thoughts with permission.
In addition, you may wish to look at getting books on teens and divorce to help her and for further reading for yourself.
Let me know how your chats go and how she responds.