Fodder hit it right, it's an introduction not a re anything. Siblings raised together form a bond, as do dogs from different origins. But siblings who've been apart for awhile are complete strangers to each other. No different than introducing dogs that have never met before. Though my personal approach is less micromanaging than some, it is always best to let them meet on neutral territory first. Take it slow, and watch closely their body language...don't rush anything, because positives can come slowly, negatives are not quickly forgotten!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain