If your grades are that bad, you'll probably be on academic probation with your chapter. They'll be interested in working with you to get your grades up. I agree with Hannah XO about the social activities-- these are events that you should only think about after you have made your classes and study time your main priority. That does not mean that you can't have fun time or spend time with your sisters. This just means that this is not the year for you to be Suzy Super Sister, but if anyone expects that out of you, their priorities are misplaced. Check with student services at your school. They may have study skills classes that can help you if you are having difficulties transitioning from HS to college. The style difference may be part of your problem, or it just may be that you had too much fun last semester.
Look for all the resources available to help you succeed. Good luck!