I slept with my girlfriend’s sister 5 years ago

Me and my gf now trying to bto and get married soon after 3 years of dating. I knew my gf got a younger sister but I have never met her or seen her pic as my gf is a very private person even on social media. Last week, my gf brought me to meet her family for the very first time to discuss about our marriage. That was when I first met her sister and realised she was the one I ghosted when I was 18 yo after we slept tgt. I ghosted her because she lied about her age on tinder. I won’t disclose her sister’s age but it was a one time thing and I felt horrible after knowing the truth.

I pretended that I didn’t recognised the sister during the dinner and kept calm so that my gf won’t suspect anything. Everything went well until later that night I received a text msg from her sister, asking if I rmb what happened 5 years ago and asked why I ghosted her. She also said that she couldn’t stop thinking about me and felt very hurt after I ghosted her.

I just told her that I was only looking for hookups on tinder during that time and got hurt because she lied about her age. I also told her to keep this a secret from my gf and just let it go as it was 5 years ago where we were still young and not know better. She agreed that we should let it go. This conversation happened earlier this year, so it has been awhile.

But recently, my gf has been asking me about my past hookups/rls. This make me panic a lot because I told her before about my past, so I don’t know why she is asking it now. I suspect her sister told her smtg because my gf keep asking me what exactly did I do 5 years ago when I was in my dating app phase. Do I come clean to her and tell her about her sister as I only had one hookup during that phase and ntg more? Or do I just say I had one hookup and not tell her it’s her sister? What should I do? Pls send help.