Ditto NavyHoops.
I noticed you posted same for USMA, so regardless of where you might be, be a good plebe: academically sat, no conduct or performance problems, pass the PFT first try.
You may have earned a weekend (overnight liberty) through one of the various ways or through - crossed fingers - a big Navy win. Plebes don't normally get them. Then, it will take a special request chit through your chain of command to overcome missed classes and presumably being well out of the area on a weekend.
Don't bring this up during PS or the first few months in your company. Learn the lay of the land, work hard to build a reputation as a good plebe who is not struggling on any front. Around November, with full facts in hand, proposed travel plans, have an informal chat with your squad leader on when the best time to drop your chit would be, and best strategy overall. It will probably be January, when you will have a new squad leader (COC changes each semester) and will know what your class sked is, how many classes you will miss, and your first semester grades are in.
When chits like this arrive on a deciding officer's desk, he/she will look at what's right per regulations, what's right for the mid (restriction? probation or unsat in any way? current GPA?), what the chain of command has recommended, what has been past practice for plebes and close family member weddings, what precedent is set. This last is key. Your request must be considered in the light of all future requests for plebes with similar records and similar reasons.
As to uniform, if your sister wants you to be in civilian clothes, then that will be part of your request chit. During leave periods (Thanksgiving, winter, spring, summer and certain other between-training periods), you rate civilian clothes. Liberty, no. You would probably leave Yard/travel in uniform, since you wouldn't have civilian clothes with you.
