Idk if I should tell my gf the truth

Me(m16) and my gf(f17) met thought my sister and cousins. She was my sisters best friend. My sister went off to college in another state last summer and me and her bsf started talking over over fall and started dating in the winter. We met and exchanged info at a dinner after homecoming. I also lost my virginity to her(that will be relevant later). I like to bully my sister about the facts I’m dating her bsf but one day she started asking me if me and my gf had intercourse and I told her we did after I told my gf I wouldn’t tell her. Fast forward to today I’m driving over too see my sister and my gf asked me to promise I won’t tel my sister anything. I said I promise but now I’m starting to feel bad about lying to her. Idk if I should tell her the trust or keep lying to her