Boy problems

Okay so me and this guy have been hanging out for like two months. We hangout almost every night. We go on Dq and Perkins dates all the time. I go with him to work on the racecar and even go to the races with him. He even let me drive his truck.  We have gone to the bar before and he told me to say that we were dating if anyone asked because I was his DD and under 21.  I'm friends with his sister though. He's scared that she's going to find out because she will freak out. I don't know what to do.