Halakhah su Deuteronomio 21:16

The Torah (Devarim 21:16-17) commands us to give a firstborn male a double portion of his father’s estate.²A full presentation of the details concerning the special entitlement of the bechor appears in Shulchan Aruch C.M. 277 and Pitchei Choshen 8:2. Thus, if the deceased was survived by five sons, the bechor receives two-sixths of the inheritance, and the other sons each receive one-sixth.

And because of this, they, may their memory be blessed, said (Bava Batra 126b) that if the bequeather commanded and said, "My son shall not inherit me," or "My son x shall not inherit with his brothers" or "My daughter x shall inherit me," when he has a son, or similar to these words; his words have no force, as he does not have the power to uproot the word of God, Who said that the heir inherits the bequeather. And even though we said that his properties are in his hands for all of his desires, the matter is to say that he may give them to whom he wants and to do whatever is his desire - and even to destroy his property - with any expression, except for that of inheritance; as this statement is against the statement of the Omnipresent and His decree. As He stated that the heir inherits, and hence a person has no power to say [that] he shall not inherit. And this is [the meaning] of our teaching in our Mishanh in [the chapter entitled] Yesh Nochalin (Bava Batra 130a), "One who says, 'X shall not inherit with his brothers,' has said nothing, for he has stipulated against what is written in the Torah. And so [too,] one who says, 'Y (stranger) shall inherit me,' when he has a daughter, or 'My daughter shall inherit me,' when he has a son, has said nothing, for he has stipulated against what is written in the Torah. Rabbi Yochanan ben Beroka says, 'If he said [it] about one who is qualified to inherit from him, his words are valid, but about one who is not qualified to inherit from him, his words are not valid.'" The explanation is if he said about one son among his sons, or one daughter among his daughters that only [that] one among them inherit him, his words are valid. And it is expounded in the Gemara from that which is written (Deuteronomy 21:16), "And it shall be on the day that he gives his inheritance to his sons" - that the Torah gave the father permission to bequeath to whom he wants from those qualified to inherit. And the law about this is decided like Rabbi Yochanan regarding common [children], but not regarding the first-born; about which the father does not have the power to uproot the inheritance from the first-born in this matter.

And it is practiced in every place and at all times by males and females. And one who transgresses it and commanded - whether he was healthy or on his deathbed - that the one eligible to inherit him, not inherit him, has nullified this positive commandment, provided that he commanded so, with an expression of 'inheritance,' as we have said. And [this is true] even though his words have no substance, as we have written above at the beginning of the topic. And Ramban, may his memory be blessed, wrote that Rambam, may his memory be blessed, skipped two commandments, one positive commandment and one negative - and both of them are about the firstborn. As a man is commanded to recognize the firstborn by giving him double, and that is a positive commandment on the father that Rambam skipped. And so too, It warns him about transferring the rights of the firstborn from him. And about this it states (Deuteronomy 21:16), "he may not make into a firstborn the son of the beloved, etc." And the teacher mentioned also did not itemize this warning [as a negative commandment], but rather grouped it all into the commandment of the law of inheritances.