I'd go the chelsea route, most sisters want what the other one has, so twins should want it moreso. you could/should/will end up having them both. for a while atleast, but while you're doing that line up another girl (backup), you can never have too many backups.
If she says "i was right, i knew you only wanted my sister" and she will, girls love being right, and they really can smell fear too, clear your mind, go to your happy place, think about lasers even.
You see, her being a girl she wont stop at that, she'll go on and on and on and on, you don't have to listen to that, just stay in your calm place till her lips stop moving, then quickly when you get the chance say, "no, but after the fuss you made about her i wanted to see what the big deal was, and you're right, she's great in comparison to you" then make up an excuse to leave, or just leave. But don't say that unless you know you can get out of there on the rapid. It could/should/will get ugly. She'll be pissed, maybe even upset. Could try to throw something at you as you walk away (so be careful of that) but during the calm will probably want to try prove you wrong thus the having of both at once. sisters at the same time is hard work, not impossible but hard work, and you WILL* be found out eventualy, that's where the backup comes in handy.
Girls although complex are pretty simple, specially** at that age, well at any age, truth has no meaning and they'll allways see it their own way. How often do you hear a girl say she was wrong??
You probably will just have to wait it out until she comes to her senses.
