RAD and sibling groups?

This is a hard question to answer, as it depends so strongly on the children involved. My situation is very similar to what you are describing, except I have the oldest 2 kids from the sibling group of 4. The youngest 2 are placed together in a separate home. In our case, the FM had the 3rd child, as you do, and a few months later took the youngest (who was under 2), as well. She then tried to take the oldest, expecting to eventually have all 4, and ended up disrupting him because he was not safe to have around the younger 2. She never had the 2nd oldest placed with her. We ended up placed with the 2 oldest a few months later. Honestly, of the 4 kids, the youngest has the most severe behavioral issues. He is one tough kid to deal with. And it was not necessarily immediately obvious, since he was so little when he was placed. I believe he has very severe RAD, though I do not know if he has been officially diagnosed. I did not get to see how the younger 2 behaved together before they were placed together, though, so it may be different from your situation. For my 2 kids, I question how "bonded" they really are. In many ways, I wonder if they would have done better as only children, because having their sibling around has allowed them to maintain a "brothers vs. parents" mentality in many ways. They don't "need" to bond to us as long as they maintain their weird, kind-of-twisted "bond" with each other and their past. At the same time, they lost their biological parents and, for the most part, their other two siblings, so...