My foster/adopted Son has 3 other siblings

We all live near by and are making an effort to help the children form bonds. The children are 1 month boy (foster/to adopt), 2 years boy (adopted), almost 8 boy and 14 girl. (both raised by my mother, guardianship). The boys are doing great, the two middle boys play like they've always known eachother, and they like the idea of the baby boy. As you can imagine the 14 girl is not terribly interested in any of this and hates her birthmother (justified). She is in therapy and I am sure part if it is her age also. I am not going to involve myself with her biomom stuff, but I would like to help her feel more comfortable with her brothers. Any suggestions? Their birthmother is in a half way house, has a sponsor, and is doing well for the first time in, I cannot remember when. As long as she is doing well we are allowing the boys to have suppervised visits. Her older daughter will have nothing to do with that. I am leaving that up to her therapist and my mother, while remaining a friend/aunt type figure for her. Any suggestions or observations are welcome.