What is the ruling on the money bequeathed by the paternal aunt to her deceased brother's children for building a house for them, given that the condition of building has not been met?

If her brother's children are her heirs, then a bequest to them is religiously prohibited unless the heirs permit it, due to the Prophet's (peace be upon him) saying: "There is no bequest for an heir." The bequest becomes valid if the heirs permit it. However, if they are not her heirs, then a bequest to them, as long as it does not exceed one-third, is valid.

A bequest conditioned upon construction is valid, but it becomes void if the condition is not met. So, if the beneficiaries refuse to use the money for construction, they have no right to it, and the bequeathed money is to be divided among the heirs.