My daughter had the opposite scenario -- two bigs. She really bonded with one, and had a nice relationship with the other, but they definitely haven't been as close and aren't in ongoing post-graduation contact like she is with Big #1. Big #2 went on two different study abroad semesters, which was one of the reasons my daughter got two bigs -- Big #2 knew she had plans to be gone for two semesters and this gave her the opportunity to still have a little.
I also had two littles, but not from the same pledge class. One, I am still tight with and we had a lot in common. I had a perfectly nice relationship with the second, but it wasn't the same "really hit it off with lots of things in common" kind of deal. This will be OK. Right now, she's your lifeline, but you'll forge other friendships in your sorority and those will just take some time to develop.
And, your twin might be a bit protective. Just keep taking the high road and breathe deeply. It's early days.
