Not sure what to do. Im in my mid 30s, I have a sister who is a freshman in college, 19 years old. I just found out she has been having a relationship for the last few months with one of her old high school teachers, a 31 year old male.

She stupidly got herself into a situation where my parents were about to find out, so she made up some story about a boyfriend at her school.

Now I have to figure out what to do about the relationship and about the lies to my parents. I don't know if I should tell them or let it go since she is legally an adult.

I have told her that if she wants to keep communicating with him, she needs to tell my parents the truth. If she doesn't feel comfortable doing that, then she needs to stop talking to him. I told her in my opinion, it feels wrong and unacceptable and the relationship needs to end. She said she sees my side and will stop talking to him, but I don't really trust her.

I feel very protective and angry and want to tell my parents and have a big talk with her about how wrong the relationship is and how she can never talk to him again. At the same time, I don't want her to feel ganged up on and betrayed and like she can't talk to me.
But I am also torn because I feel like my parents should know and I hate lying to them. I know they would be very disappointed in me if they found out I knew and hid it, but I also know they will be completely devastated if they find out.

Any suggestions are appreciated - also, not sure if I'm just completely overreacting either...