Is the stepmother entitled to claim a sum of money that the deceased had promised her before his death and then spent on something else, especially with evidence to that effect?

The money that the father intended to donate to his wife, but she did not receive it until he died, is a right for all heirs according to their legal shares, and not exclusively for the wife. This is because a gift is not binding until it is received, and it becomes void upon the death of the donor before its receipt, as mentioned by Ibn Qudamah. However, if all adult and discerning heirs agree to donate this money, or part of it, to the father's wife, this is a praiseworthy act of kindness and there is no sin in it.