Is it okay to date someone my sister went on a date with 7 years ago?

So, I had this huuuuge crush on this guy in high school. I was obsessed with him. We talked and had fun together but things never really panned out. Fast forward 7 years later, he dm’s me and we start to catch up again. We talked for a month before we finally go on a date and things took off from there. I decided to tell my sister about the new guy I started dating and I found out something troubling. My sister said that she had went on a date with him in high school. I never knew this until now. I asked him about it and he admitted they went on one movie date and she had gave him head (this was around the time me and him stopped talking in high school). I didn’t know how to feel. She knew I had a crush on him and went out of her way to message him and ask him out. After talking about everything with her I decided to keep talking to him given it was almost a decade ago and they never dated and she didn’t care that we were talking. Things have been great with him and I and we started developing strong feelings. About 5 months after me and my sister talked, she texts me saying she isn’t comfortable with me dating him. I was shocked. I listened to her and told her that it’s not fair to make me chose and I’m finally in a healthy relationship and was happy. I considered leaving him and asked my mom and friends for advice. They all said it was fine to date him especially since I had talked to her beforehand. My sister ended up blocking me and refused to come to any family gatherings if I was there. I feel like i’m being forced to choose and it’s really hurting me, I really love this guy but I love my sister too and I hate that she’s making me choose. I feel like either way I lose. What would you do?