Autism & Siblings

I would like some advice from anyone that has taken a placement from foster care where the child with autism was placed first--given enough time to settle, then a bio-sibling was placed. I am trying to make an informed decision and my areas of concern are that I have suspicions that something has either happened or that triggers PTSD episodes in the child with autism, there is a family history of domestic violence that the siblings enact (at the level of children, but this child with autism can't make that distinction) and issues of being a "favored" child. I understand the important of sibling placements---and normally, I wouldn't have any doubts. But relationships are different with autism. So how did you handle it?