One who was breastfed by your wife's sister is the maternal uncle of your sons and daughters

Question: I have a wife, and I have eight daughters from her. She has a younger sister who is fifteen years younger than her. A man was breastfed by their mother, so he became her brother.
But my problem is: my daughters say that he is their maternal uncle through breastfeeding, and they uncover their hijab in front of him. I forbid them from doing so, but they refuse. I ask for your guidance. May Allah reward you with good.

Answer: If the man mentioned was breastfed by your wife's mother or by her father's wife, while she was under her father's guardianship, for five breastfeedings or more, while he was within the two-year period, then he becomes the maternal uncle of your daughters through breastfeeding. It is permissible for them to uncover in front of him, just as with other mahrams, and to be in seclusion with him; because the Prophet (peace and blessings be upon him) said: It is agreed upon that it is authentic. This is provided there is no suspicion that would prevent seclusion with any of them.