Having a terribly difficult time......

By way of background here is my story: We have been married almost 2 years. After 3 months of being married, we took in my cousin's 5 month old who was already in foster care. She turned 2 over the weekend and it is looking like she will be adopted by April!! :) In February, 2010, we took in two foster children (I was pregnant at the time). They are sisters and were 10 and 13 at the time. The 10 year old was our flower girl as we were really close with the foster mother. Suddenly in January the foster mother died, leaving the girls with no where to go. We took them both in and were never informed of ANY problems with the 13 year old. In April, I gave birth to our first biological child. He is now 6 months old. Within weeks of having the 13 year old, we started seeing extremely expolosive behavior. She will throw, kick, scream, and carry on for hours. She needs uninterrupted, undivided attention otherwise she will go into these rages. We have taken her to multiple therapists, done intensive treatment with a local hospital, and had her on various meds. It seems that she has reactive attachment disorder. She refuses to do anything by herself - including cleaning her room. She will try and sit on my husband's lap all night if he let her. I could go on about the damage that has been caused to my house. During one episode she threw a ball and it hit my son's swing (he was in it at the time). We have been trying to work with her as much as possible. Tonight, we are scheduled to go to an attachment therapist. We were asked last week if we were going to adopt, but we do not have an answer. We have delayed the 10 year olds adoption as we do not want to adopt her first without the 13 year old as it would cause extreme havoc in my house. The sisters have been separated, we know find out, due to this type of behavior. We have been told by another therapist, that it may not be the best idea for them to live together. They are both legally free. She has been trying hard to work on her rages and, to some degree, they lessened. There are no longer 6 episodes a week (they used to last for 8 hours or so). She still has these episodes and wakes up the 2 year old and 6 month old. The 6 month old is aware enough now that he turns his head when she is kicking, throwing, etc. When she goes into a rage, it is impossible to keep her away from the rest of the kids. She wants to be in the room that you are in, but will throw, kick and scream. I simply cannot tolerate that when I am trying to comfort a 2 year old and 6 month old (and worry about the 10 year old's reaction to it all). It has been a tough decision for me, but I am ready to let her go and hope that a family with less or no children will open their home to her. I am scared for the safety of my other children. My husband and I both work full time and I am just not sure that we have the time and attention to devote to her. (She just needs a TON more than anyone else). Thoughts, suggestions, experiences? I just need to know that its okay to let go. I know it will be incredibly difficult, but I dont know what else to do.