What is the ruling on the mother of the wife appearing with her hair uncovered before her daughter's husband, and the wife appearing with her hair uncovered before her husband's father?

The wife's mother is a mahram to the man, and his wife is forbidden to his father, according to Allah’s saying regarding forbidden women: "and your wives' mothers" and "and the wives of your sons who are from your own loins."

It is permissible for each of them to uncover her hair in front of her mahram, but this is forbidden if there is fear of fitna (temptation) or lust, as stated by An-Nawawi.