A commandment to fulfill the wishes of the deceased

I am speaking in a way that the father said he wanted to write a will but did not have time, and revealed that he also wanted to give to the daughters.
From the perspective of the son, it is held that he succeeded him, but since the laws of the country divide the inheritance equally, the daughter can trace the signature that she waives her share, and in any case it is found that the son will receive only a proportional share, and the daughter claims that she is carrying out the will of the deceased, and therefore is not obligated to make restitution for the loss.
As it seems to me there is some ra”a in Sheot a Sach that says that there is an obligation to honor the father with the father's money, the question is whether the inheritance is considered his money, or because he succeeded him is considered the son's money?

This has already been raised here before. If one holds to the method (accepted by most jurists) that state laws have no standing in relation to inheritances, and that Torah law is the determining factor (because it is a prohibition and not money), then the money legally belongs to the son, and the daughter must sign so as not to usurp it. If one holds that state laws also have standing in relation to inheritance, then the situation is different, of course.
Assuming that the money belongs to the son, the question arises of a mitzvah to fulfill the words of the deceased. See an overview here:
https://he.wikipedia.org/wiki/%D7%9E%D7%A6%D7%95%D7%95%D7%94_%D7%9C%D7%A7%D7%99%D7%99%D7%9D_%D7%93%D7%91%D7%A8%D7%99_%D7%94%D7%9E%D7%AA

And here you will see a discussion of whether this is a moral obligation or property ownership:
http://www.daat.ac.il/mishpat-ivri/skirot/151-2.htm