Legal Question in Family Law in Pennsylvania

No. The fiancee's mother has no rights to the child whatsoever. You do not indicate who the other parent of the child is. Is it the fiancee? If its the fiancee and she has visitation with the child then I don't see anything prohibiting the fiancee from letting her mother see the child while she has periods of custody unless there is something in the custody order/agreement setting forth that the child is not to be around the fiancee's mother.

If the child is your child from another relationship with another woman, then fiancee's mother has no rights at all ever unless you permit her to be part of the child's life, get married and later divorce the fiancee.