Sister problems

So my sister did one of the most horrible thing to her boyfriend she cheated on and they were engaged and she didn’t even say sorry to the guy who is the nicest person on the planet and the worst part HE said sorry when he did do anything and no one is telling her she was wrong everyone is like he wasn’t a real man he wasn’t giving you attention even tho he had gotten you Gifts and she cheated on him on his Birthday he god dam birthday and he was telling me how much he loves her and she’s the best and I don’t know what I would do within her she broke him into pieces and I feel so bad sorry I just had to rant about my fucked up sister