Things like this usually start out as a bit of fun. However, if you play your cards right, then it could turn into something more serious, like a relationship. The key is to not overcomplicate things and go with the flow for now. She wouldn't invite you if she's not interested. But perhaps she herself isn't yet sure where things are at between you two. The holiday might be a great chance to find out. I would suggest accepting the invitation, because I can't see a girl asking someone to spend a holiday with her unless she really likes him and enjoys his company. I wouldn't ask her yet how to define your "relationship". If things click on the holiday and you two have a good time, then it may be right to pose the question about becoming exclusive. At the early stages, its nice just to enjoy each other's company. Once things get a bit more serious, then you can start defining your relationship. Be careful though that you don't send her mixed signals - just continue the way you've been doing till date.