I’ve been fucking my friend’s brother

So, a bit of context… Me and my boyfriend have been dating for almost two years, he’s mormon, so the church says 16, his parents say 18… We’re both minors, and his sister (my friend) is 19 currently and we’ve been friends for 2 years Since he’s not allowed to even date yet, we thought it would be best if we just didn’t tell anyone we were dating, especially his sister because we were scared she’d tell. I was allowed to sleep over at their house because his sister is my friend, and obviously we wanted to hang out and have sleepovers yk? Me and her tell each other everything, which is why I feel so guilty not telling her, but I just feel like if we told her now, that she’d get mad and tell their parents or something. But the thing is that we’ve been getting pretty serious. The first time we had sex, it was a complete accident, but not anymore, and I feel terrible about not being able to tell her. Should I tell her now? Or wait until he is allowed to openly date.