Domestic violence and harassment

Hello, sir My sister got married in 2010. at time of marriage my father given all households thing, money and gold as asked dowry to my sister's in low's but after few days of marriage their in-laws harassing my sister for small small household reason, and started torturing her unnecessary, her husband worked abroad that time and he never stops or asked his family for their behaviour, instead of resolving the issue he also joined to misbehaving with my sister and creat nuisance so badly that my sister bound to come home, it was her initial stage of pregnancy... However we have tried to resolve the issue within the family and given all medical and required support to her. And post delivery we sent her back to her in lows, 2-3 months gone normally and then again started the same situation. It happened three times.. Whenever she got pregnant they created nuisance and sent her beck to us. Later we understood they were scaping maternity cost and all. Please also understand they never expenced a single penny on my sister's clothes or any daily needs, we sent all required thing to her keeping that in mind one day everything will be alright. She worked at their home whole day in house hold activities like cooking and cleaning and taking care of enire family member's more then a servant and in return of that she never ever had a full stomach food yes she got beaten whenever she replied to their harsh words, here lot of thing can be said but i described in short how she suffered. Now my sister got two daughters (her 1st baby expired during delivery) and my brother in low and his family never wish to accept them now, we have tried to resolve the issue on discussing matter with both side's society and some prominent person from society also requested my sisters husband and inlows to resolve the matter and and accept my sister and babies. But they never wish to. However we are now fed up with the situation, its my sister whose life have been spoiled without reason. Now we want to take legal support to punish them, please guide me how to move with strongly.