All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
First of all, you should know that it is impermissible to delay the payment of zakah; whenever the zakah becomes due with the passage of the lunar year from the date of acquiring the wealth subject to zakah, you must hasten to pay the due zakah. If you wish to pay zakah in installments, then you should pay it in advance and not after the lunar year has passed. The basic principle is that the immediate payment of the zakah is obligatory and that it is impermissible to delay it.
The Fatwa of the Permanent Committee for Iftaa' reads, “It is impermissible to delay the payment of zakah after the completion of the lunar year except for a valid excuse such as a lack of rightful zakah recipients when it becomes due, inability to deliver it to them, or the absence of money at the time or the like...”
We have previously highlighted in several fataawa that zakah must be paid once it becomes due and that it should not be delayed. Please refer to fataawa 86980 and 14605.
It is permissible to pay the zakah to your brother if he is among the rightful recipients; for example, if he is poor or in need and cannot find a job to provide for himself or is physically unable to work and earn a living. In this case, it is permissible to pay your zakah to him. The poor and needy who are entitled to receive the zakah are those who do not have what meets their basic needs: food, drink, clothing, house, medicine, and the like.
The same applies to your sister, her husband, and your uncle. If they fit the description of those entitled to zakah (they are poor or needy), it is allowable for you to pay your zakah to them. In fact, they are more deserving of your zakah in this case because paying zakah to them would be considered an act of charity and kindness to relatives at the same time.
You do not need to inform your relative that you are giving him zakah money, but you must transfer full ownership and possession of that money to him to dispose of it as he wishes. You do not have the right to stipulate that he repays you this money when his situation improves.
Also, it is not allowable to buy a house from the zakah funds. Shaykh Ibn ʻUthaymeen held that the preponderant view in this regard is that zakah funds cannot be used to buy a house for the poor. However, he mentioned that this matter is open for ijtihaad (scholarly reasoning). It is more prudent not to buy a house for the poor from the zakah money. If your sister's house is very small, you can give her family a sum of money that pays the rent of a house for a year from the zakah funds. The majority of the scholars held that the poor person should be given what will suffice him for a full year from the zakah funds because it is paid annually.
You cannot pay your zakah to finance a business for your sister's husband. Rather, you give the zakah to him, if he is among the rightful recipients of the zakah, to dispose of it as he wishes, because transferring the money to his possession is a condition of giving zakah.
Also, it is impermissible for you to lend the zakah money to someone because it is not your property; rather, it is a due right of the rightful recipients of zakah. Hence, it is incumbent on you, dear questioner, to pay the zakah immediately to its rightful recipients, and you cannot pay it to a relative who is not among the rightful beneficiaries of the zakah merely to win them over or do them a favor.
Allaah knows best.