In brief, is it permissible to mortgage land to buy my uncle's land, with the mortgagee utilizing the mortgaged land without rent until repayment? And is it permissible for my mother to bequeath to one of her married daughters due to her difficult circumstances, given the dispute among the siblings regarding the permissibility of this bequest?

The aforementioned mortgage transaction is not permissible because it is an interest-based transaction. This is because the owner of the money lends it in order to benefit from the produce of the land, and every loan that draws a benefit is usury (riba). Mortgaging was legislated for security, not to be benefited from in exchange for rights. As for the mother's will to her daughter, it holds no weight and is not enforceable after her death unless the remaining heirs consent to it, and all of them are mature and adult. Otherwise, it will not be executed or be valid, based on the saying of the Prophet, peace and blessings be upon him: "There is no will for an heir unless the heirs wish it."