When I started the adoption process, I didn't see how anyone could pull any child away from their birth siblings. It seemed very cruel to not allow them all to grow up together, even if it was in a completely different family.
I have gradually changed my views on this over time and while I admit that separation will be tough to handle, I am convinced it is a much better way to help the adoptee heal, especially those with a solid memory and attachment to their birth family.
I have heard that those who catch crabs can keep them all in an open bucket, without a lid, since the crabs in the bucket will actively pull down any crabs that start to escape their confinement.
While I am not certain if this is true for crabs, since I don't catch them, I know it was true in our family. In fact, more and more of the "pulling down" gets exposed as all my children are adults and more and more leaks out about what happened when they were children, both from them and as I analyze things from a different perspective.
None of them could really heal, since they all had each other as their "family" and could actively keep out the "interlopers" who would break up the family and keep them from their rightful parents. Sure, the adoptive parents might keep them for a few years, but they would all be adults soon when they could return to their proper family, at least that is what they likely thought. Of course I am extrapolating their thoughts here, but this is what they all have done, to one extent or another.
Even the younger ones, with less of a connection when adopted, have joined back in. Our youngest says she is not interested in the birth family, but she ended up living up there for a while and could possibly end up going back at some point, in spite of all her proclamations to the contrary. That pull is so strong, nothing can break it, especially when it is reinforced during the entire growing up time.
While I may not have convinced you to break up sibling groups, perhaps you can be a little less harsh on someone who does that. Keeping them together is unlikely to get them to like you any more. In fact, doing so is more likely to get them to gang up and have them all reject you, instead choosing even a dysfunctional "family" they return to if they can reconnect with those who gave them birth.