Is it permissible for someone who sold shares in an interest-based bank and wishes to purify the money (tazkiyah) to buy a house for his divorced daughter and her child with this money, knowing that her ex-husband does not provide for them?

It is not permissible for a person to give his Zakat to someone whose maintenance is obligatory upon him. The maintenance of a poor daughter is obligatory upon her father until she marries, and if she divorces, her maintenance reverts to her capable father. It is also not permissible for a father to give his Zakat to his daughter's son who is not sufficiently provided for by his father and mother, because his maintenance is obligatory upon him. The majority of scholars agree on the obligation of maintenance for a poor grandchild, even if he is not an heir.