Is it permissible, in Islamic law, to gift a property to one's sister's children on condition that the donor benefits from its rental income for life, and that they are not to dispose of it until after the donor's death?

Registering a property in the name of the donee for them to take possession of it after the death of the donor, without them possessing or disposing of it during the donor's lifetime, is not considered a valid gift (hibah) but rather a will (wasiyyah). A will is a man gifting his property to another person after his death. A will to a non-heir is valid up to one-third of the estate. Any amount exceeding one-third is not enforceable unless approved by the heirs. Sisters' children are not among the heirs, so a will made in their favor is valid up to one-third.