Is it permissible for my brother, whom I had suckled, to conclude a marriage contract between his daughter and my son?
Such marriage is impermissible since one`s nephew in this situation is considered an uncle to one`s daughter; consequently, he is the brother of her father by suckling. In such cases, the people in question should go to court to prove the suckling.
Is it permissible for a woman to ride a taxi without a Mahram (unmarriageable kin) in order to attend a gathering of knowledge?
It is permissible for the woman to pursue the knowledge that she needs in order to perform the religious requirements due on her if there was nobody to teach her at home, provided that there is no temptation involved. Moreover, we don`t recommend that she rides a taxi without a Mahram although such an act isn`t from the forbidden Khulwah (seclusion).
Should nail polish be removed before making ablution?
Yes, it should be removed because it prevents water from reaching the nails.
How are gaps filled during congregational prayer?
A praying person is allowed to take two steps in order to fill the gap in the row ahead. If the distance was long and requires much movement, then it is impermissible for one to move to fill it because much consecutive movement (three movements) during prayer invalidates it.