Sister forgiving a cheater

My sister keeps forgiving her dumbass boyfriend and she won't listen to any of us. Today he tried kissing some girl in their school on the stairway and the girl messaged my sister telling her and even sent screenshots of proof of him saying not to tell my sister it happened and that it can be their secret and shit and I told my sister she needs to move on and she said "I want to forgive him. Mistakes happen"