What can we do if my sister faces constant abuse?

My Sister married to a man in 2017, it's been 7 years but there marriage is always in hurdles, there are constants fights, verbal abuse, abuse her family(my mother and me), he always complained to her saying we didn't give them much in dowery, multiple times we have asked elders to come in between and settle their marriage but it's not happening, they fight on every peculiar reasons and then she travel to my home and stays, now i'm also fed up with this as this is keep happening, Most recently, he found out she is involved with some other man, and found photos in her phone, and kept her phone, she is crying, and asked me to book car for her so that she can travel to my home.