They don't have to look alike nor act alike to be siblings. I look nothing like my half brother and sure as hell don't act like him, and we're still related.
A lot of people find Celestia to be a mary-sue with no personality. I completely disagree with that statement 100% though. Celestia is my favorite Princess.
I hate to say it, but I think it would be Twilight. Even though she's the main character, she'd be the easiest to write off; allowing for the other characters to enjoy more of the spotlight.