When does love turn into abuse?

So! I know this couple, one of them is related to me. ;/ Sometimes I fear for his life.

When he brought her home I knew her quite well. She was in my science class. We are the same age. It was weird cause around school she was known as a slut. I didn't want to believe this cause she seemed so nice to me.

It all became true to me when he brought her home to meet our family. She was wearing a short white tank-top and tight black pants. It was weird cause she called my mom "mom" that's rude and she just met my mom. I knew there was something weird about her.

So time went on and she got more moody, strange, and aggressive towards "him" She played all nice though when she was around my family. "he" would come home with bruises on his arms and face. He said it was a stick that hit him in the woods. I knew this was a lie. She was bad news. I wanted to get rid of her!! he would always call her names and that he hated her!

When does love become just abuse? Why do we hold on?

he and him is my brother! He has changed. He drinks and smokes a lot more then he use to. he is depressed. ;/ He needs to find true love somewhere else.