What case can we do?

My sister has been married for nearly 9 years now. She had a love marriage to a much younger man, about 5-6 years younger. I never liked my bro in law, he has no respect for elders or Indian sense of marriage, family or parenthood. My sister was engaged to another man earlier but that the guy broke off the engagement. Now, my sister had fertility issue and got proper treatment and gave birth to her son 6 years after marriage. I supported her during her fertility treatment but her husband didn't give any care, he didn't provide her a penny for the treatment or for other household expenses. My sister stopped working 2 years into her marriage due to pressure from her in laws to get pregnant, they provided her money for her fertility treatments because they know their son's nature and hate him. He has started an affair with a married divorcee woman who is not working. And he has started financially sustaining her as well. My father passed away around 1 and half years back and my mother is very weak in her 70s. She has advised my sister to ignore the infidelity of her marriage and stay in the marriage for the sake of her 2 and half year old son.My mother told her that she can't bear the embarrassment of her returning home, but if my brother in law crosses the limit and start bringing the woman to her house to live with her. Then we would take action. My bro in law doesn't care of putting his son in preschool. My sister came home for a few months to our hometown, at that time, my brother in law invited his girlfriend to stay in his house, after much fight and persuasion, my brother in law bought return flight tickets for my sister and her son to Bangalore where they live. On return, my sister got shocked when she saw the other woman's garments lying around his room. My brother in law has made my sister and her son sleep on the ground floor room on cheap mattresses while he bought bedroom set when his girlfriend came to live with him in those few months in the 1st floor. He has made sure the son and mother don't enter first floor or his room while he is around. He has refused to eat what my sister cooks, dinner or lunch but has morning breakfast tea.he works night shifts in BPO. So he gets up around 1 PM and after having tea goes to office, but gets home at 5 in the morning. His shift gets over at 11 or 12 midnight, but he stay with his girlfriend who lives very near to his office. During weekends of holidays, he spend the entire time with his girlfriend, whom he has made creative Head of a small photography company he launched. So, infront of others he is using the card of office work with his girlfriend. My sister has fought with him many times. He said either she bears with him if she has guts or leave him as he doesn't have an iota of love for her. My sister found a sherwani and a saree with sindoor stain over them, so she even told me that she doesn't know whether he is remarried or not. He gives the rent for the house, buys diapers for his son and buy groceries for his son but does not give a rupee to her son. My sister's in laws know everything about their son, live in relationship with his girlfriend but they don't say anything to him but they told my sister not to divorce as it would bring shame to their family as everyone will speak about their son philandering ways. My sister have not me that her husband washes his own clothes and won't let her do it, doesn't eat what she cooks and doesn't even let her touch him because his girlfriend said so, besides they have been living in separate room after their son was born. He has mentally and emotionally tortured my sister. In case of divorce, what other charges can be filed against him? How long does a divorce process take? I guess my sister and my nephew may be entitled to maintainence and child support expenses but in case of a divorce, if my brother in laws doesn't provide or refuse to provide or halt the flow of financial support, what actions can we take? Will that be another snail paced legal battle? And How long will the actual divorce take place?