Is the failure to implement the aforementioned will considered a sin, and what is the legal ruling regarding this detailed will?

The father's will to marry off young children from the inheritance is not obligatory to execute, because marriage is a fulfillment of a need, and these children have not reached the age of marriage. Therefore, it is not permissible for the heirs to execute it unless the mature, discerning individual among them permits it. If the father married off his older children who needed marriage and had no money, then he is not obliged to give the rest of his young children who have not reached the age of marriage the same amount he gave the older ones. If he willed the older children to marry off the younger ones, this will is not obligatory to execute because it does not command an obligatory matter. However, if the will is executed, it is a good deed for which the children will be rewarded.