Hello,
Thank you for posting your question.
Since the mother of the child is no more, that means you, the father, are the natural guardian of the child.
In order to obtain the custody and guardianship, you may file an application under the Guardianship and Wards Act. And legally also, under the above Act, being the father, you are entitled to get the guardianship of your daughter.
Additionally, in order to create pressure on your in-laws, you may file an application under section 97 of the Cr.P.C. also stating that your daughter has been wrongfully confined by them.
However, the correct procedure is to file an application under the Guardianship and Wards Act.
Do let me know if you have any further query.
Regards,
Nishant