Our characters were adopted by antibenders because they didn't want to chance the gene being passed down as a carrier gene. So they adopted children with the hopes that they would be antibenders as well. These two kids can be actual siblings or just kids who knew each other at the orphanage. They can have a good or bad relationship. That really depends on what kind of characters we crank out. These two kids, much to their new parents' dismay, are actually amazing benders even if they didn't show it for a couple years. 

My question to you is, do you want them to be real siblings or not? That kind of affects what kind of story we're going for here. I feel like once we know our characters, we can build the story based on them. I'm going to crank out a female bender real quick and then you can make one to match her however you like. I'm fine with whatever. Just know that if you're wanting romance, they can't be siblings. xD