Does past love matter ?

Yes, I'm a purist myself and it's very hard to deal with this kind of thing. you have to remember that falling in and out of love is a natural process that cannot be helped. OK, she had relations with her cousin before she was with you, but you are the one she loves now. I don't know about your past experiences - maybe you have at least fallen in love once before, but when you move on you always completely deny to yourself that you ever felt anything for that person, so having relations with her cousin is probably the biggest regret of her life - you should be proud of her for having the guts to tell you!