Is it permissible to help a sick uncle-in-law (husband of a maternal aunt) with cancer from zakat funds to buy expensive medicines, knowing that he is neither poor nor needy, and has a moderate income?

If a sick person is able to purchase medicine with their own money, it is not permissible to give them zakat (definition), but it is permissible to give them voluntary charity (sadaqat at-tatawwu'). However, if they are unable, they are considered poor or needy, and their expenses, including the cost of medicine, become the responsibility of their able children. If that is not possible, then it is permissible to give them zakat.