There's still a chance they didn't v*! As long as you wash your hands after being around people from that party, you should be just fine. If you think your boyfriend may end up catching it, have him wash his after he spends time with them. Seeing as you were very careful about washing your hands and keeping them away from your face while you were around his family, it doesn't seem very likely you will catch it. Bring ginger candies, ginger ale, hand sanitizer, disinfecting wipes, sea bands, and/or anything that helps keep you comfortable and feeling safe during the trip. Have fun on that trip, you will be okay.

Edit: I might add I was in this situation a few months ago. I kept my distance from my boyfriend and his family for four days, and disinfected anything he might have touched when he was at my house previously. Nobody else in my boyfriend's sister's family caught it, and neither did I.