Should i tell about the cheating?

my younger brother who is 16 im 18 has a 14 year old girlfriend she is pretty but its her personality that everyone loves. shes real nice and sweet but my brother is a tool. he literately talked her into having with him and she didnt want to she wanted to wait but he said hed break up with her so she did. she is in love with him but i know for a fact hes cheated on her with 3 other girls. I want to tell her but i feel id be betraying my brother also my younger sister and 3 of my cousins are best friends with her but they woont tell her because they are afraid of my brother. I feel sorry for her shes real nice and i dont under stand how he can be that way. just the thought of my girl cheating on me makes me want to cry, i wouldnt be able to take such betrayal so what should i do tell or keep it to myself