Is it permissible for a woman to sit with her brothers-in-law?
It is impermissible for a woman to sit with her brothers-in-law, and it is also impermissible for a person to have a Khulwa (seclusion) with his sister-in-law.
Does the 'aqīqah count as valid if it is slaughtered before the seventh day from the birth?
All praise is due to Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
The time during which it becomes permissible to slaughter the 'aqīqah begins from the moment the newborn is fully delivered from its mother's womb.
If the animal is slaughtered prior to the birth, it does not count as an 'aqīqah — it is simply considered an ordinary sheep slaughtered for its meat.
And Allah Almighty knows best.
Does using a respiratory device (oxygen) affect the validity of fasting?
Using an oxygen device does not affect the validity of fasting because oxygen is a gas with no physical substance.
However, if medicinal substances with physical particles are added to the oxygen, its use would break the fast, as it enters the body cavity through a natural passage.
What is the ruling on performing voluntary (nafl) prayer during the time of istiwā' (the sun's zenith) on the day of Jumu‘ah (Friday), and is this ruling specific to someone who has come to the mosque to perform the Jumu‘ah prayer?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
It is permissible to perform voluntary (nafl) prayer during the time of istiwā' (the sun's zenith, i.e., its highest point before noon) on the day of Jumu‘ah (Friday) — whether for someone who is attending the Friday prayer or someone who is not, and whether they are among the people obligated to attend it or not.
It is stated in Mughnī al-Muḥtāj (Vol.1/P.310): "The more correct position (al-aṣaḥḥ) is that prayer is permissible during this time without restriction — regardless of whether the person will be attending the Jumu‘ah prayer or not." And Allah, the Exalted, knows best.