My sister and I were caught masturbating by our mom

We are both male and female twins. We just turned 18 and are getting ready to go off to college together. We have always been extremely close, it used to creep people out when we were younger. We never did anything sexual until we were 16, we always just saw ourselves as siblings. One night we went on a trip with our mom to a lake house. We were walking in the dark down a path but had got lost. We found a spot in the dark where we were able to play with each other. We were having sex, maybe for 5 minutes. When we finished we heard a voice. “How long have you two been doing this”. It was our mom. We tried to explain how we aren’t ashamed and we’re sorry. But she said we didn’t have to be sorry but she was just shocked. Then she said she had always wanted to, but was just too scared. We said since we’re all adults why don’t we? So we had a threesome with my mom. Ever since then we’ve had sex multiple times. But we still haven’t told our dad. He’s a very strict person so I don’t think he would be okay with it.