I'm getting a lot of cognitive dissonance with a potential hookup, and am looking for some serious viewpoints/guidance.

Chapter 1:

Junior year college. Start flirting with a friend of an ex girlfriend, who I had been digging for a while. End up staying at her house for two nights of a road trip with my buddies. We bone, I continue on with my trip. New girl leaves to study abroad a week after and has since been traveling for two years. We both move on and have no feelings for each other any more. She ends up telling my ex and they have a fight about it. Gets resolved and everybody is seemingly okay.

Chapter 2:

Current day, college grad. Browsing Instagram and see a babe (10 in my books, completely my type). Follow her, like her pictures for a few weeks. Learn that, holy shit, it's abroad girl's younger sister by two years. She messages me and says hi. One thing leads to another and we're texting and calling each other. She's a short 20 minute drive away. I'd been digging on her hard for a while, so the situation is unreal.

I'm into this girl, but there are very obvious things to consider before we hang out (and probably fuck).

  1. I banged her sister two years ago, and she probably doesn't know about it
  2. I'm empathetic because I'm really into this new girl and she seems to have a wonderful, caring nature about her. Feel like I'd feel bad internally for not telling her about her sister and I's fling.
  3. She's hot as fuck and super cool.

How would you handle this?