2nd marriage done by govt. employee without divorce from 1st wife

Hi Sir, My younger sister's husband (govt.employee in court) did second marriage without any obligation. he threat my sister (married in 2008 and have 2 daughters) and took her signature for no objection for 2nd marriage and he did 4 to 5 days ago in jaipur. We are basically from ST (meena caste) category and do not have any kind of money power/support, as we are a lower medium class citizens. Also my sister's in laws were aware about all the things and they also support to their son for the second marriage, as this the very bad thing for us now. Would request you to please give us best advice for this criminal offence which is done by my sister's husband and suggest for further steps, so that my sister does not loose her family and right, neither my sister and her daughters can suicide, as they do not have anything with them. Please please Sir show us bright side of future, so that my sister can save their family and respect. Waiting for your reply. Regards, Rahul Meena