Dear querier,
I have read through your concern and have answered below to the best of my abilities. I sincerely hope this information is helpful to you and solves your issue. thank you.
As of now the natural guardian of your sister's child is your brother-in-law however under the Hindu minority and guardianship act the court Is of the opinion that even if the act gives the power to be the guardian of a child if that person is not being a good guardian and is not maintaining welfare of that child then you can challenge that in court.
You can file an application with respect to the guardianship off your sister's child to the District Court which has the jurisdiction in the place where the child is currently residing according to the Section 8 of the guardianship and wards act of 1890 you are entitled to apply for an order if you desire to be the guardian of the child or if you are a relative of the child and you satisfy both the criteria’s.
Ultimately the welfare and the proper taking care of the minor of the child is of the most importance to the judiciary to the judge so you can submit your application and they will accordingly move forward with it I hope the information I have provided helps you. Thank you.