Congrats on your bid!
I'm also one of those girls that took a while to fit in, and am still having a bit of trouble. The older sisters are close because they've had time and memories to get that way... you'll be getting your fair share! If you have sisters in your dorm, get ready for events with them or just keep your door open to chat! You'll be having more than your fair share of new member activities as well.
One of the things that helped me the most was getting to know girls on an individual basis. Living in the house helps as well -- I had a huge conversation about religion today with one of the girls in our spring '12 pledge class and came to realize we had a very similar background. It was really nice to learn that! It does take time and effort, like every friendship, but you'll get there in the end
