Hypothetical situation

[Hitchyhero]

Me and my partner adopted last year and we are having discussions if we want to adopt again. We think it's too soon atm anyway, and thinking a few years down the line.

Another factor is cost. We arnt on great income (but not bad either) and with a second child, it'd certainly be tight (but doable).

Then a discussion happened about what would happen if our sons Birth Mum had another child that went up for adoption but financially we were not ready. I'd feel terrible that we would have to pass our sons sibling. So we got to both wondering if it's at all possible to Foster the sibling of your adopted child for a while? Does this ever happen?

I doubt it, though every situation is different. But if the plan is adoption (including foster to adopt) then the SWs will be looking for parents ready to adopt. If the plan is not (yet) adoption, then foster parents would usually be expected to facilitate contact, which would be tricky if that's the adopted child's birth parents.

Never say never! But my guess is unlikely.

Of course, IF that situation ever occurs, you could always ask about financial support for the sibling. But again, given budget cuts, if the child is easy to place an allowance is unlikely.

@Hitchyhero

I'd agree with Allington
Why worry about something that may never happen. Enjoy the child you have and don't spend your time thinking about what would happen with a child who does not exist. The birth mum may keep subsequent children, what about child 3 or 4 or even 5. She might not have any more children.
Focus on what you have, the future has a habit of taking care of itself.