This is quite a complicated and long story so I will start at the very start -5 years ago.
So from 15 to just before my 21st birthday I was with a guy let's call him Joe. It was a pretty toxic relationship at times and I ended up breaking up with him as he was all I had known my adult life I struggled not going back and was very lonely. I had at the same time began speaking to a boy on my university course who said he would be there for me and help me get through the breakup. I naively began to channel my heartbreak through him and we hooked up a couple of times. Looking back it was obviously a rebound and we were both still friends. Fast forward a few months and he starts dating a girl and I'm dating a guy but we are still friends and his girlfriend invites me to a party at him and I check with him he says to come. So I go to the party with a mutual friend we have a great night and his brother happens to be there. I have an immediate chemistry with the brother let's call him Frank. Frank and I hit it off and so begins a wonderful relationship. We have now been together 5 years and are getting married next year.
I obviously told him briefly and in no detail about the brief hook up with his brother months before we even met which he said was fine and we moved on. He has only even mentioned this once in our relationship.

So my problem, Frank the love of my lifes brother has had trouble maintaining any form of long lasting relationship he cheats on his girlfriends hes had 2 children from different women in the nearly 6 years I've known him yet he gets away with everything. His most recent girlfriend and mum to one of his kids has recently forgiven him for an infidelity and when we were speaking she basically came out with 'he even told me about you and how you were obsessed with him after one hook up and you only got with his brother because you couldnt have him- if he can talk about you like that his own sister in law what hope is there'
I was literally gobsmacked - he had completely re written history and made it sound horrendous. It really bothers me as I would never want frank to think he was ever second best as that is completely wrong. I will admit during the few weeks after my ex I was needy and I kicked off at him once for getting with someone in front of me but that was more my heightened emotions after my break up. Obviously I regret ever hooking up with his brother but if I hadn't I would probably never have met Frank so it's a double edged sword. I want to confront his brother privately but not sure if it's worth it. I just hate thinking either that's how he sees me or hes a pathological liar.

Any advice? What would you do?