Is it permissible to give zakat to one's sister and her husband for the purpose of buying a house, knowing that they are employed and their salaries barely cover their living expenses, and they own a plot of land that they cannot afford to build on, and its value is insufficient to purchase a ready-made house?

If your sister's husband's salary is not enough to cover the rent and other expenses, it is permissible for you to give him zakat (definition) to cover his needs for a full year. The fact that his wife contributes to the expenses does not prevent this; her contribution is a donation from her, and it is permissible for her to give her zakat to her husband.

As for buying him a house, the majority of scholars hold that a poor person should only be given enough to cover their needs for one year, so they should not be given money to buy a house. However, the Shafi'is and Hanbalis, in another narration, are of the view that a poor person should be given what will move them from poverty to wealth. According to this view, it is permissible to give him zakat money that enables him to buy a house to shelter himself and his family.