What? Even though it's not about that, as you write further on, what is this person saying? Is an adopted sibling more or less on your side? Or is that the responder doesn't feel s/he can get on the side of an adoptive sibling? The comment made no sense in the context of the quote.

BTW, I kind of identify a bit. My youngest sister always got away with pretty much anything. She didn't get beaten or even supsended. And, she was a right little termagant.