1. Well, ask your sister what does she want out of this marriage.
2.If she does not mind ill treatment of your parents then you better forget this and ask them not to visit her in laws any more nor meet their any dowry demands.
3.You or your aprents can do nothing if your sister does not want to disturb the marriage.
4.However if your sister wants she can lodge complaint u/s 498A,406,504,506 of Ip c for the torture and harassment done to her and her parents.
5. However in that event her marriage may be jeopardised.
So decide accordingly.