I've [27M] been dating my girlfriend [23F] for two years without realizing her sister [26F] is my old FWB. Yesterday we visited her at their parents house and she demanded I not tell my girlfriend. However, I feel like I owe it to my girlfriend to tell her. What should I do?

Sorry for the confusing title, this is kind of a weird situation. Basically, in college I was really into the party lifestyle and had a number of casual sex partners. One of which was apparently my girlfriend's sister Kate, who I'd see a few times a month. We'd basically just get high and have sex, no real feelings or conversation or anything. Eventually for no real reason we stopped seeing each other, and I all but forgot about her once I graduated college.
4 years after that I had moved to a different town, where I met my current girlfriend Sarah through a work function. We've been dating for about two years and it's been going really well. After the new year we've finally both been able to take some time off, and we decided to visit her parents and her sister who live back in my old college town.
Well yesterday, after we had gotten settled in, Sarah's sister Amanda approached me and asked if I had told Sarah about us. I said I didn't know what she was talking about because I just met her. After some confusion it came out that Amanda is Kate and she had given me a fake name back then. I also didn't recognize her because she put on a ton of weight and has a radically different style now (colored hair and piercings). She literally looks like a different girl, and this was the last thing I was expecting
On top of that she has demanded that I "respect her privacy" by not telling my girlfriend about our past. However, I feel like my girlfriend would want to know and would be really mad if I didn't tell her. On top of that, I feel like there's something else going on because "Kate" told me back in the day that she went to my college, but she's apparently an alumni of some other state college and never went to the college in town. And while I didn't recognize her on social media, she's had to have recognized me and has even commented on some of Sarah's pictures of us, so she's at least been lying to Sarah about knowing me.
I'm torn, do I respect Amanda's privacy or do I tell Sarah about our past? There might also be some fallout because Amanda has clearly been lying to her family, so if I am going to tell Sarah when should I do it? We're supposed to stay until next Friday, and Amanda lives with her parents so revealing this could make things awkward for everybody if I do it while we're still staying there. How can I navigate this situation while causing the least amount of drama?
TL;DR: I've been dating my girlfriend for two years without realizing her sister is an old FWB. She lied about her name back then and her appearance has changed dramatically over the years, so I never recognized her on social media. She's demanded that I not tell my girlfriend about our past, but I feel like I owe it to my girlfriend to tell her. Additionally telling my girlfriend might reveal a bunch of other lies that her sister has been telling over the years, and I have no idea how damaging that could be to her family. What do I do?


Source.

Hmmm...tough situation

Try to ask ur pastor

It's not a big to tell her or to keep it, the ball is on your court.

Just keep your mouth shut...not for her lying ass sister, but for your relationship.