When a child needs the most care, protection and emotional support, if his parents are not around, then in my opinion, the person who is by his side at that time and alleviates his fears and suffering is the one who actually plays the role of a guardian. Because while biological relationship is important, guardianship has to be proven through responsibility, presence, love, sacrifice and continuity. No one can claim the full status of being a guardian just because they gave birth.
Waooo this is just the truth about such kind of life experience. Sure being biotically related to someone doesn't make that person a share holdeer in the life of the person. From birth to at least teenage hood, close monitoring in essential to the child, because during this stages , the child's destiny and future is framed and sharpened. So if attention is not giving to this child he or she during this stage they might become something wors which the said parent wouldn't want to relate with because of the life style of the child.