AdoptUSKids is a great resource for foster and adoptive families. They did a FAQ about adopting sibling groups:
I think the big thing for us is letting go of the fact that there will be more children. We have to focus on the three that we have and not bring them back into the foster care realm by taking on their newborn brother (not that he’s in foster care yet). I hope in the future when the children are older that they would consider opening our home to short term or respite care. It would need to be a unanimous vote though.