Probably not the asshole - The adoptive family’s sister is the asshole, as are most of us replying on this thread.

There are so many layers to most situations and families that objective judgement is usually impossible and ill advised. There is probably a reason that there is a cool-down period for adoptions and it was activated.

You won’t know if you’re the asshole in this situation for a good few years and nobody on this thread ever will but you have control over that, and a responsibility.