Beauty is subjective. You shouldn't get offended or upset by one person's perception because there is always someone else who will think differently. To be honest, you both look too similar to really make a definitive judgement. I can guess you're the left simply because your avatar picture shows you don't have bangs, the girl on the left doesn't, whereas the girl on the right does. Assuming both pictures were taken before any hairstyle changes occurred, of course. Outside of that, I would say left because you introduced the picture as me and my sister, not my sister and me, which for the left to right readers means, you're on the left, but that would be more of a stab in the dark rather then actually using my head since people use bad grammar far too much for that to be a reliable method any more.
I also notice that she's obstructing her face (and no one likes to show themselves with an unclear picture)
Not always true, but generally a good method as well. At any rate, if this were a game, there would be a lot of clues to make figuring it out easy.
Lotta stupid stuff. Weekend was freaking sweet. Had me some fun times. You?
Not a great time, but no complaints either. Took the girls with me to San Francisco because they had never been and I really think they need to get out of So.Cal. more to expand their views on the world around them. So needless to say, I spent more time keeping track of them while they went crazy on the town with my cousins then having fun myself.
Maybe I should've dragged you out of your cave too, Arizona boy. =P Please tell me you at least watched your kids open up presents or something? Not like you don't have plenty of opportunities to accidentally keep a few on the truck. 
Are you going to join the modders vs pros games?