I was searching in my Uni library for something totally unrelated, came across this (first book on this Amazon link) and thought of you. No idea if it's any good, though, as not read it. The first one on the link is a bit dear, but the second by the same author is more reasonable.
www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=brothers%20and%20sisters%20in%20adoption