1. Do you have in your possession the suicide note which your sister wrote before committing suicide in 2011? If yes, then you may file the case subject to offering a legally plausible justification as to why the case was not lodged by you for 3 years. Your brother-in-law and his family members whose names have been mentioned by your deceased sister in suicide note may be arrested.
2. He could not have taken the child without a court order empowering him to do so. Immediately file a case for child custody.
Needless to add, in depth consultation by you with your lawyer should as a matter of legal prudence precede the filing of both these cases so as not to leave any escape route for them.