Just to bring a second side to this discussion.
Skiing with your bros may be fun, but you have to look at her view too. Does she really want you to come with her to the family thing in SC? If she really wants you to go with her, then maybe you should consider sacrificing the ski trip. If she is okay with you not going then go ski. The biggest thing I can see in this dilemma is choosing between you or her.
I would recommend talking with her, be honest. say that you want to go on the ski trip, but that if she really does want you to come to SC with her, then you will go. Usually in this case she will say to go on your ski trip. If she doesn't, then you have a "get out of jail free card" for the next time you want to go do something. Just remember to be honest. She will probably appreciate your honesty; whereas, if you lie and say that you can't get out of the ski trip and she later finds out you where being dishonest with her the repercussions would probably much worse.
TL;DR: Be honest.