Guardianship for children as not a reason for deeming a person their Mahram

Q: My maternal grandfather died when my mother was about four or five years old. Before his death, he entrusted his cousin, Muhammad ibn `Aly ibn Mastur, with the guardianship of his daughter. We only heard about the verbal will he left, as it was not written. According to this will, the said guardian is to act like her father and his wife is like her mother. Allah (Exalted be He) orders women to wear Hijab (veil) in the presence of any non-Mahram (not a spouse or an unmarriageable relative), so please advise me concerning the following: Is it permissible for my mother to unveil her face in front of that guardian and his children? Is it permissible for my brothers and me to be in the presence of his daughters while they are not wearing Hijab? Is it permissible for me to consider his children my uncles? It is worth mentioning that my mother was not breast-fed by his wife; he only raised her in his house until he gave her in marriage to my father. She did not live very long with his children as she was much older than them. I hope that you can explain this issue in detail so that I can explain it to my mother. (Part No. 17; Page No. 399) May Allah safeguard you.

A: This person's guardianship of your mother does not allow her to unveil her face in front of him or his children. Also, her sons are non-Mahrams to this person's daughters. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.