Bava Basra Chapter 8, Mishnah 6. The starting point of this Mishnah is that a person is given special credibility, legal reliability, to claim that someone is his son. This is a unique law: generally, when a person claims without proof that someone is his brother, his words do not have many ramifications as long as we do not assume they are true. However, when a person recognizes someone as his son, his words are given special credibility.
The source for this law in the verse:
At the end of the passage dealing with the firstborn, it is stated: "For he must acknowledge the firstborn, the son of the unloved wife, by giving him a double portion of all that he possesses". The verse deals with a context where the father supposedly seeks to deprive his son of his birthright, and it says "acknowledge" (yakir) - he knows exactly who he is, and he must give him a double portion of all that he possesses. The word "yakir" is seemingly superfluous in this context, and from here we learn that the father is believed to acknowledge his son and to claim that a certain person is his son, and this claim carries special weight.
Two areas where this credibility is relevant:
Inheritance: A person passed away and left two sons, Reuven and Shimon (and let us assume his daughter is the eldest, so there is no law of a firstborn here), and on his deathbed he declared: "I have another son, Pierre in France, your brother from a previous marriage." From now on there are three sons, and even though there is no proof that Pierre is his son, based on that testimony before two witnesses, he will be entitled to a third of the inheritance. This is a unique law that does not apply to other relatives: a childless person whose uncles inherit from him is not believed to say, "I have another uncle named Pierre in France," and his words have no significance.
Yibbum: A person passed away without any descendants - his widow requires yibbum or chalitzah with his brother. Our Mishnah deals with someone who had no children, and seemingly no brother either. If he declared on his deathbed, "A certain person, Pierre, is my brother" - his words carry no weight, and the woman is free and is not obligated in yibbum with that person.
The text of the Mishnah:
"Ha'omer zeh beni ne'eman" - One who says, "This is my son," is believed. This credibility has two ramifications: the son will inherit like the other sons, if there are any, and the wife will be exempt from yibbum, since the deceased left an heir (assuming that child is alive after his father's passing).
"Zeh achi eino ne'eman" - But one who says, "This is my brother," is not believed, neither regarding the division of the inheritance nor regarding the obligation of yibbum. Nevertheless, the Mishnah continues: "Venotel imo bechelko" - and he takes with him in his portion.
Let us illustrate this: there are three brothers - Reuven, Shimon, and Levi. Reuven claims that Pierre in France is their brother, while Shimon and Levi reply that they know nothing about this. Their father passes away and leaves twelve thousand. According to Shimon and Levi, there are three brothers, and each takes four thousand, while according to Reuven, there are four brothers, and each takes three thousand. Therefore, Shimon and Levi take four thousand each, and out of Reuven's four thousand he must give one thousand to Pierre, since according to his own claim he is only entitled to three thousand, and Pierre is entitled to three thousand. The thousand comes, therefore, exclusively from Reuven's portion, while Shimon and Levi are not obligated to give anything.
"Mes yachzeru nechasim limekoman" - If Pierre passed away during the lifetime of their father Yaakov, it turns out that only three brothers remain, and everyone agrees to this. At that point, the entire equation falls apart: the portion that Reuven intended to give to the alleged brother is not given, and each of the three brothers takes four thousand.
The novel concept in this law is that one might mistakenly think that Shimon and Levi would claim against Reuven - "If you claim that he is our brother and that the thousand passed to him, then we are entitled to a third of that thousand." This is not the case, since everyone agrees that at this stage each one is entitled to four thousand, and there is no reason why the other brothers should not receive their portion.
"Naflu lo nechasim mimmakom acher yirshu echav imo" - If Pierre passed away, and as far as we know he also claims, like Reuven's claim, that they are brothers, and let us assume for the sake of discussion that his only relative is his supposed brother, Reuven. Since Shimon and Levi are also related to Reuven, when Pierre's estate passes to Reuven it is divided into three equal parts: a third to Reuven, and two thirds to Shimon and Levi. Here ends the first part of the Mishnah.
A Deathbed Will:
The Mishnah returns to discuss how inheritance works. In the previous Mishnah, we learned that if a person leaves instructions on his deathbed, even orally alone, that he wishes to give a gift - his words stand as if he made a formal transaction. Here the Mishnah teaches: "Mi shemeis venimtzeis daytiki keshurah al yereicho, harei zo einah klum" - a person who dies and never made an oral declaration, but it was found that he wrote a deathbed will and tied it to his thigh. No one disputes that he wrote it, that he signed it, that it has witnesses, and that he placed it on his thigh so it would be found - and nevertheless, it has no halachic significance.
The reason for this is as follows: if the daytiki says, "I want my properties to pass to my friend Pierre in France," there is no substance to this. His sons are his heirs, and they take the properties, and the fact that he wrote and stated this before his death does not change a thing. The special Rabbinic enactment allowed a terminally ill person to make an oral declaration, but here there was no oral declaration, only writing, and therefore it is not binding.
"Zikah bah leachair, bein min hayorshin bein she'eino min hayorshin, devarav kayamin" - however, if while still alive he wrote the daytiki, that will giving a gift, and handed it to another person to acquire it on behalf of Pierre in France - it makes no difference who receives the document for the purpose of executing the transaction: a relative or a non-relative, a son or a non-son - his words stand. The gift given in the daytiki is binding, since a written document handed over as a formal transaction is no less binding than an oral statement.
Thus, when a terminally ill person writes his will in order to grant his money, and even all of it, to Pierre - a random person in Paris - and hands it to another, and that person receives it on his behalf to make a transaction in his favor, Pierre acquires the gift while the giver is still alive, and is entitled to everything, while the heirs will receive nothing in such a case.