Younger partially sisters husband committed 2nd marriage

For a lawyer hailing from Darbanga, you may look for one from this forum or visit the bar assocation of Darbanga.

Remember that if your sister is not cooperating then you cannot file any case on her behalf because she may express her unwillingness to conduct the case before court then the court will dismiss the case.

In that case all your efforts will go in vain including loss of money due to advocate and court fees.

Lot of thx sir 

but can't it's a possibility that all members of his husbands family might have pressurised him and mentally made him brain washed

No answers can be given to your guesses. 

We can look for remedy and relief only to the subsisting problems and not for imaginary concepts.