Break Up & Divorce

Can anyone make sense of this?

The ex girlfriend of my sister's husband cheated on him 2 years into their relationship and he broke up with her and cut all contact. Yet for some reason, my sister holds this grudge against her for cheating and calls her all sorts of names and she even tried to sabotage her husband's friendship with his high school best friend, because the best friend still keeps in contact with her husband's ex girlfriend. Mind you my sister's husband has no problem with his best friend being friends with the ex girlfriend who cheated on him. My sister on the other hand doesn't like it and she makes it known she absolutely hates the woman and she says she hates cheaters.

Here's the thing that makes no sense to me. My sister's own son ( my nephew) was in a 4 and a half year relationship. He even lived with the girl who was like family to us all. The son cheated on her and got another girl pregnant. Without hesitation, my sister not only supported her son despite cheating and hurting the girl, but she opened her home to the girl he cheated with.

How can you hate and despise your husband's ex girlfriend and claim you hate cheaters, but see absolutely nothing wrong with the fact your own son cheated and got another girl pregnant. Yes the girl who my nephew cheated with was aware he had a girlfriend when she slept with him. Everyone knew he was in a relationship. It was even posted all over social media.

Can anyone make sense of this?
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