How can we seek maintenance for baby and wife?

My sister got married to guy in 2013. Post marriage, she has been emotionally and financially tortured by her husband and in laws. So after 3.5 yrs of tolerating all dat, she finally gave up and left her matrimonial home and came to her paternal home wid her 1 yr old kid. We wanted to settle things and send her back but due to her husbands financial mess due to meagre income and extravagant lifestyle that includes buying expensive sunglasses, shoes, smartphones, car,etc. But as a father and husband he has minimal duties towards his son and wife which he clearly refuses to do. My sister has been asking for her minimum rights in the form of little amount to just take care of her baby and herself but everytime she does so he fights or shows aggression which makes is determined that we cant send her back to him as he and his family would do more emotional and financial torture. So we want to send him a notice seeking monthly maintenance. Pls advice the steps