Sweetheart4u,
Put the big sister that you want on your list first. I did just that, and got her even though she wasn't taking a little. She spent the next week telling me how she was going out of town and wasn't even going to be at the Revealing. Boy, was I surprised!! Unfortunately, we aren't close like I had hoped.
But, it goes to show you, that if you really want to have a Big Sis your New Member Coordinator will try to make it happen.
On another note, last semester new member class at my chapter was really large and several of them were matched with their second or third choices. (And in some cases, lils were placed with big sisters not on their list.) It has worked out. The most important thing to remember is what others have said already on here, in the end
everyone is going to be your sister!!!!!! 
Some are going to be closer than others. And you can always be adopted!!!
Let us know how things turn out!!
Meredith