If she cheated multipule times will she cheat again? Can a guy get back with her just cause his desparate for sex?

My dumb twin brother got back with his cheating ex.(He brings her over to sleep and I'm so sad) It breaks my heart that he trusts her like nothing happened. Will she cheat again? They together for 4 months already and seems like she doesn't cheat but I don't trust her. Should I ask guys I know to get on with her so he"ll see that she's a cheater or should I just wait for her to cheat on him once again?

If she cheated multipule times will she cheat again? Can a guy get back with her just cause his desparate for sex?
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