where was the marriage performed, has it been registered under the Hindu Marriage Act. If so, and in bangalore, then you can ask your sister to proceed to file Criminal cases against her husband, mother-in law and her husband under the provisions of the Dowry act.
Is your sister staying at her in-laws house after marriage or she is staying with your parents.
It is better to file RCR in the family court, thus enabling your sister to obtain permission to make candidate to agree to bring your sister to USA.