Why are women so terrible at seeing which men will abuse them and so good at detecting men that give them the ick?

My brother is constantly an asshole to his and makes her cry multiple times a day sometimes. She was staying at our place bust just quietly went home this morning and he is just continuing to sleep. What is she seeing in him that im not that is apparently worth being treated terribly for? Whenever she is over im hearing screams and crying.
Im already doing what i can to move out but she literately doesn't have to live through this. Foids gonna foid i guess.