Is it permissible to give an apartment designated as a "Maqra'ah" (a place for قرآن recitation) – based on the mother's will – to a divorced sister to live in temporarily, knowing that the sister currently has enough to rent a place, but will need the apartment after retirement, provided that the apartment returns to being a Maqra'ah after her death?

An endowed apartment cannot be disposed of except as specified by the endower. Therefore, your sister is not permitted to live in it, whether she is poor or rich. The supervisor of the endowment must use it for the purpose for which it was endowed, and it is not permissible to leave it unused. If the benefit of the endowment becomes defunct, it is permissible to sell it and purchase another apartment with its price to replace it in the endowment, or to spend its value on a similar cause if the designated cause becomes defunct.