Siblings and Parents sucked through a dimensional rift, captured on the other side by a race that has destroyed humanity in that reality, parents killed.

siblings auctioned to different owners as exotic species pets/slaves, end up on different planets. 

GOAL: get back to earth, and get to their own reality.

SIbling Specs:

Brother:  more applicable "real-world skills" 

Sister:  More of an assassin type character sneaking around in the shadows