Is the maternal uncle Islamically entitled to return the money he took as a trust from his deceased sister (the mother) with the aim of "making something for her children," without fulfilling his prior promise to give them a plot of land from his property, this being after 18 years and a rise in the land's price?

If your maternal uncle stated that you have a dunnam (a unit of land) in exchange for the money your mother gave him, as a definitive and immediate transaction, then this is a valid sale, and he is not permitted to revoke it, even if the dunnam is an undivided share of a known piece of land. However, if this was merely a promise, or if the land was not specified, then the sale did not occur or is not valid due to uncertainty (jahalah). In this case, he owes you the amount of money he took from your mother, and if the currency's value has decreased by one-third or more, he must compensate for the full depreciation if he took the money to invest it for himself.