Crankin, in addition to what OakLeaf said, I would continue to ask about how the kids are doing in coversations with that family. Yes BIL has feelings, but what is actually happening is to the girls, gifting bone marrow is not like donating blood, and the child with leukemia needs all the support she can get.

Also, is there a way to encourage the sister who's donating (and with the weight problem) to be more active as she recovers from the bone marrow donation? Getting her into healthy activities would help with her weight as well as dealing with personal issues and that her sister is sick (sick kid will get more attention than other kids).