Cynical me here:
I think that just sometimes, social workers, like anyone, can struggle with how little power they actually wield. And the only people that allow SWs a little power are adopters. And sometimes, when insecure, that leads to a little willy-waving, sorry, I can't think of a sex-neutral term. The kids don't like them, the birth parents don't like them, the only people trying to please them are the adopters. And when you're under pressure, some people slip into bad habits, and want to feel powerful.
So they like to remind you that they have the power.
We have met many great SWs. And one, who I think was under a lot of pressure, who let me know that she has the power.
It leaves a very nasty taste, doesn't it?
But they have to have good reasons to not place the sibling with you. They will have to document why this baby was not placed with its sibling. And 'we have families that want babies' is not a reason.
Good luck, you gluttons for punishment!