How can Ibn Baz's fatwa permitting a woman to sit with her husband's male relatives in the presence of a mahram, with the assurance of no fitna (temptation/strife) and adherence to the legal hijab, be reconciled with the prohibition of mixing (ikhtilat) – even if fitna is assured not to occur – mentioned in other sources?

The forbidden mixing (Ikhtilat) is the lack of adherence to the legitimate Islamic guidelines for the presence of men and women in one place. These guidelines are: the legitimate hijab, modesty, absence of Khalwa (private seclusion), not softening the speech, and lowering the gaze. The husband's relatives are like other non-mahram men. The prohibition of their entering upon women has been narrated in the saying of the Prophet, peace and blessings be upon him: "Beware of entering upon women." A man from the Ansar said: "O Messenger of Allah, what about the Hamw (male in-law)?" He said: "The Hamw is death." If their sitting with the woman is restricted by the guidelines, then there is no harm in it.