Section 9 of Hindu Adoption and maintenance act says:
Persons capable of giving in adoption.-
(1) No person except the father or mother or the guardian of a child shall have the capacity to give the child in adoption.
(2) Subject to the provisions of sub-section (4), the father or the mother, if alive, shall have equal right to give a son or daughter in adoption:
Provided that such right shall not be exercised by either of them save with the consent of the other unless one of them has completely and finally renounced the word or has ceased to be a Hindu or has been declared by a court of competent jurisdiction to be of unsound mind."
In your case since the biological mother is not alive, based on her death certificate an adoption deed in favor of adoptive mother can be made and registered after which she will be the child's legal adoptive mother. You can consult a local advocate for the procedures to be adopted in this regard.