What is the ruling on paying Zakat on money that reached the Nisab, but was spent on purchasing a car, with the remaining amount supplemented by a loan? Is Zakat obligatory after the debt is repaid, and is it permissible to give Zakat to the daughters of one's sister who live with him?

Reaching the nisab alone is not sufficient for zakat to become obligatory. Rather, the passage of a full Hijri year (hawl) on the wealth is a condition. So, if the wealth is spent before a year passes on it, there is no zakat due on it. However, if it is spent after a year has passed, its zakat has become obligatory and is not waived by spending it. There is no harm in giving zakat to needy and poor nieces, even if there is someone who voluntarily spends on them.