I said I DO understand why it would bother him. I'm not denigrating his feelings at all. It would probably bother me, too. But I'm not sure it would be a deal-breaker thing.
There are so many things that can keep you from loving someone, so many challenges: infidelity, age, emotional problems, distance.
The OP wants to spend his life with this woman. He and she have something very special. I don't think he should throw it all away because she has some baggage that involves a relative. It is in the past and neither one of them can change it, so why not move on and focus on what they have together in the now?