Why women protect abusive men?

My sister husband is verbally abusive to her and he screams like a mad man and he also is doing this to my niece. I tell my big sister baby you got to leave. She fights me on this gets so mad at me and protecting him at all cost. I also have reason to believe her husband is molesting their daughter. I’m hurt she won’t leave and I can’t understand why she protects him? What can I do? My husband says leave it alone and let god work on it. I told my husband I wouldn’t be a real sister if I didn’t try to save her. What should I do? And why does this happen in abusive house holds?
Updates
+1 y
She is my older sister by the way
Why women protect abusive men?
Post Opinion