This may not help with your trip next month, but think about it for the future. When my husband and I traveled to Europe, we had a home away from home (not to mention local tour guides) in France, Germany, and Denmark.
Hosting foreign exchange students gives a family the opportunity to learn about other cultures and the possibility of visiting them in the future. Just think, when your French sister gets married one day, guess who will be on the guest list?
Bonus: you’ll already have some experience with French culture from living with her for 6 months.