I just want to say that there are sororities on campus that most girls will fit into. When I rushed, of course I dressed up for every event. I'm a big fan on wearing sweatshirts and jeans to class, especially when it gets cold here during the winter. Very rarely do I wear makeup to my classes (unless there's a hot guy in there!

). However, during the week I rushed, I made sure I looked good going to class every day. You never know what girls are checking you out around campus.
Think of it this way. It's one week of going through rush where you have to look pretty cute every day. Anyone can handle 5 days! And don't worry about your non girly-girl type personality. Once you're comfortable with all your sister and they are comfortable around you, you'll see how much of a closet dorky side a lot of girls have!