Is it permissible to give one's wife money from the Zakat so that she may buy an oven and a heater for her sick, needy brother, knowing that the appliances will be registered in his name and that the wife will use them with him, and without informing him that they are for him, for fear that he might sell them and spend the money on unlawful things?

Zakat (definition) requires transferring ownership to the rightful recipient. Therefore, it is not permissible to give it to someone under guardianship except through their legal guardian. If the person is not under guardianship, it is not permissible to give them Zakat unless ownership is transferred to them. It is permissible to purchase devices for them with Zakat according to those who permit giving the monetary value unconditionally or for a beneficial interest, such as the view chosen by Ibn Taymiyyah. To prevent them from selling the devices, they can be advised, admonished, and warned using appropriate methods.