you need Court Order appointing step father as a Legal Guardian of child .The courts are empowered to appoint guardians under the Guardians and Wards Act, 1890.
Section 7 of the Guardian and Wards Act deals with the power of the Court to make an order, as to guardianship and it reads as follows:
"7. Power of the Court to make order as to guardianship.-(1) Where the Court is satisfied that it is for the welfare of a minor that an order should be made--
(a) appointing a guardian of his person or property, or both, or
(b) declaring a person to be such a guardian, the Court may make an order accordingly.
The Hindu Minority and Guardianship Act is supplementary to and not in derogation to Guardians and Wards Act