[AOII Angel]

I have a different experience with twins in my chapter since we had two sets of identical twins in my pledge class as well as one set in the senior class when I pledged. All of these sets were extremely close like you describe for your own relationship. Also, my older sister (less than a year older) is my sorority sister. We always shared a room growing up and throughout college and frequently compared ourselves to twins. I couldn't imagine being in a different sorority than my sister. My pledge sisters who were twins said they couldn't either. That being said, establishing our own "identity" wasn't a big deal for any of us.
You'll know whether or not you want to be in the same chapter when you get to know those chapters better. If you really do want to join the same group, make sure you guys tell the chapters that y'all both want to join the same group if at all possible (but make sure they know that you will still pledge if you get a different chapter so they won't think you'll drop if they pledge you but don't get your sister....enough is up to chance in the process that you can't take this thought for granted!)
As for a way to stand out a separate people, only you and your family/friends really know the differences, so think about the differences this summer so you can have something to point out (make sure whatever it is will be a positive for both of you!)
After living with one of the sets of twins from my pledge class (at different times NOT together!) I now know that they were VERY different people even though no one could tell them apart!

Good luck...you'll figure it out as you go!