Praise be to Allah.
The maternal uncle has no guardianship over his sister’s daughters if their father is still alive
The maternal uncle has no guardianship over his sister’s daughters if their father is still alive, is of sound mind, is accountable and is present. It is well known that care of children is a duty on parents so long as they are alive, and they are the ones who are responsible before Allah, may He be exalted, for taking care of them and bringing them up.
It was narrated from ‘Abdullah ibn ‘Umar (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Each of you is in a position of responsibility and each of you is answerable for those under his care. The ruler of the people is in a position of responsibility and is answerable for those under his care. A man is in a position of responsibility over his household and is answerable for those under his care. A woman is in a position of responsibility over her husband’s house and children and is answerable for those under her care. The slave is in a position of responsibility over his master’s wealth and is answerable for it. Each of you is in a position of responsibility and each of you is answerable for those under his care.” Narrated by al-Bukhaari (2554) and Muslim (1829).
But the maternal uncle has a responsibility with regard to enjoining what is right and forbidding what is wrong. So if he sees his sister’s children doing something wrong, he must rebuke them.
It was narrated that Abu Sa‘eed al-Khudri said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Whoever among you sees an evil deed, let him change it with his hand (by taking action); if he cannot, then with his tongue (by speaking out); and if he cannot, then with his heart (by hating it and feeling it is wrong), and that is the weakest of faith.” Narrated by Muslim (49).
An-Nawawi (may Allah have mercy on him) said:
With regard to the Prophet’s words “let him change it,” this is in the sense that doing this is obligatory, according to the consensus of the ummah. The obligation to enjoin what is right and forbid what is wrong is in accordance with the Qur’an, the Sunnah and the consensus of the ummah. It is also part of that sincere advice (naseehah) which is Islam.
End quote from Sharh Saheeh Muslim (2/22).
Does the maternal uncle have the right to discipline his sister’s children?
In a situation such as this, the maternal uncle does not have the right to smack his sister’s children, because disciplining them by smacking them is only the right of the father or the one who has guardianship over the child, as appointed by the father or by the judge of an Islamic court.
It says in al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah (10/25):
It is permissible for the father or mother to smack their child by way of disciplining him, and the same applies to the one who is appointed as the child’s guardian…
As for the teacher, he may obtain the authority from the child’s guardian to discipline the child. End quote.
But if the father knows what the maternal uncle is doing and keeps quiet about it, then this is approval on his part of what the uncle is doing, so it is like delegating the uncle to educate and discipline them as he sees fit.
In some families, the maternal uncle has a status that allows him to discipline the children, and this is a matter of custom that people acknowledge and do not object to it.
But if the father objects to what the maternal uncle is doing, then the uncle should limit his involvement to offering advice [to the children], and he should also offer advice to the father and mother, because this is their responsibility regarding their daughters, and they are the ones who have the authority to prevent their daughters from wearing haraam clothing by force.
And Allah knows best.