I obviously only know a few details about your situation and how close the bond you have with your sister really is, but I will give you these general pieces of advice:
You're going to college. There are a myriad of opportunities presented to you. Every person is interested in different things, and 99.9% of the time, people can find at least one thing that will make them happy outside of class. Find what makes you happy (on your own) and take advantage of all that college has to offer.
Be yourself! Don't get caught up on what some people might think of you (whether it be sorority chapters or anyone else), because for every person who shoots you down, there will be five more who will become friends.
Joining a sorority is for life. Having a (biological) sister is for life. You can choose the former, but not the latter. You obviously love your sister, and you know that no matter what, she'll be there forever. If you join a sorority, it will be too. Make sure that if you're accepting a bid, that you're not sacrificing your own happiness for your sister.. or anyone else for that matter. And don't let her do that for you, either.
If you do end up being sorority sisters, that'd be wonderful, and I'm sure you'll have the time of your life.
If you don't, that'd be wonderful, and I'm sure you'll have the time of your life.
Remember that it's not the end of the world. Maybe it will be the perfect opportunity for the two of you to find yourselves, so to speak.
Have fun
