Is it permissible to give zakat to unmarried and unemployed sisters, and to an unemployed brother, knowing that the questioner sometimes spends on them, and does that spending prevent giving them from zakat?

It is permissible to give Zakat to a sister for a surgical operation, provided two conditions are met: that her maintenance is not obligatory upon the Zakat-giver, and that the sister does not possess the cost of the operation. This is because, in such a case, she is considered poor and thus falls under one of the categories eligible to receive Zakat. Maintenance becomes obligatory upon the Zakat-giver if he is wealthy and would be her heir if she were to pass away. There is no harm in adopting the view of the Malikis and Shafi'is, who permit giving Zakat to a needy brother or sister, as it is a recognized opinion and offers flexibility.