Hiya, my son has 3 siblings, at the moment it's only the elder one who's 9 that has issues about him, the younger one is fed up of being hit by him etc but doesn't really know why he does the things he does. The eldest has hated him, had suicidal thoughts, fed up of him being centre of attention, ruining times out, us never going out during school hols etc because of him and his violent nature, she gets the brunt of it, after me that is.
I recently bought her a book from the NAS called everybody is different and since she's read it there's been a massive turn around in her behaviour towards A, she's tried to cuddle him, been far more tolerant of him, says she feels more sorry for him now and not as angry at him now she knows he can't help a lot of what he does and she's even made attempts (good ones I might add) at calming him down when he's starting meltdown and has managed it once or twice! She seems happier than she's been in weeks and I think it's had a positive effect thankfully, before she read the book I was struggling to explain things to her in a way that she's understand.
I have also been told of sibling groups but I have yet to find out all the info on these.