Domestic Violence and dowry

Hi, This is about my sister....she got married in 2012 with a govt bank manager..... problem started there itself......during engagement ceremony her sister in law started shouting over arrangements and food.....My father and we told them to take back the gifts ....as we r not interested in this relationship.....Then they started showing guilty over it......She and her father came to say sorry......My father as a nice human being forgave n forgot......Now marriage preparation started.....his father came to my father and asked for a car....my father replied he can't accept this as he hadn't given to any daughters earlier and this is not in our family trend.....so he went to my Nana and demanded a car.....my Nana agreed.....and on marriage we gave them......After 1 month my sister got pregnant and he left her at our home....as he was posted in Chennai.....my home is in Ranchi....From then only he always wanted to leave her in mayke....even if she wanted to go he refused politely.......he and his family members always making fun of her and her dowry.... As this is not good quality......his father in law took her car and using it as his own......one time she demanded her car for one month during her last days of pregnancy.....they started harassing her mentally. Now her husband is having an affair in his bank and started beating my sister....even he is not taking responsibility if his kid too.... what can we do.....