Is it permissible to give zakat to the guardian of a legally incompetent person for the purchase of devices that will be used by both the guardian and the legally incompetent person, while concealing the ownership of these devices from the legally incompetent person to protect him from misuse?

If your wife's brother is poor and in need, it is permissible to give him money from zakat to buy a water heater and a stove. Your wife benefiting from these appliances does not matter, as they are owned by her brother.

If the brother is legally incompetent (mentally deficient or a prodigal spendthrift), the zakat should be given to his guardian. If he has no guardian, it should be given to whoever takes care of his affairs, such as his mother or sister.

If there is a fear that the prodigal spendthrift might sell these appliances, he should not be told that they are his property. Instead, dissimulation should be used to make him understand that they are yours, and you have placed them with him for him and your wife to use. This is an act of kindness towards him and a way to preserve his wealth. As for the mentally deficient person, it is sufficient for whoever takes care of him to take possession on his behalf.