Of course, my vote is for Cozumel. I'm still not in favor of AL in Cozumel (maybe other places) because of all the great restaurants and the joyful and interesting people you will meet in them - be it other tourists or locals. Your sister will be cheered just going out at night for dinner and walking around San Miguel. The Coral Princess Resort is a good location - close to town, wonderful staff, great snorkeling, excellent food on site, good dive shop, and low key enough to be quiet, yet a little fun at Happy Hour. Others really like Hotel Playa Azul or El Presidente.