What should the guardian of his sister's son, whom he did not adopt, do when the boy reaches puberty and becomes a stranger to his wife?

Thank you for your care of the orphan. This is an act of great reward, and whoever combines care with financial support for the orphan will receive an abundant reward.

After the orphan reaches maturity, the affection between you should continue. However, if he is not a mahram to your wife and was not nursed by her, it is not permissible for him to be alone with her or to look at her body, because she is a stranger to him. This is based on the Almighty's saying: "O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments. That is more suitable that they will be known and not be abused. And ever is Allah Forgiving and Merciful" and "And not expose their adornment except to their husbands." And based on the Prophet's (peace be upon him) saying: "No man should be alone with a woman unless there is a mahram with her."