Notwithstanding State law to the contrary, whenever a final decree of adoption of an Indian child has been vacated or set aside or the adoptive parents voluntarily consent to the termination of their parental rights to the child, a biological parent or prior Indian custodian may petition for return of custody and the court shall grant such petition unless there is a showing, in a proceeding subject to the provisions of section
1912 of this title, that such return of custody is not in the best interests of the child.