Child custody of 6 months old in case of mother's death

I want to know suppose at d time of mothers death the child 6 month's old is taken by mothers sister to take care, becoz the childs father ws in d state of shock of his wife's death. Later he he ws normal n started his normal life by going to work n all. When nw he is fully stable he wants d child back bt the deceased sister is nt giving her n mentally harrssing n nt allowing her to meet. Plz recommend good advice as d deceased husband love his daughter very much n wants her back.