1) "A FATHER WITH HIS SONS, AND SONS WITH THEIR FATHER"
QUESTION: The Mishnah discusses four types of relationships with regard to inheritance: relatives who inherit from and bequeath to each other; one relative who inherits the other, but does not bequeath to the other; one relative who bequeaths to the other, but does not inherit from the other; and relatives who neither inherit nor bequeath from each other.
The Mishnah gives examples of the first type of relationship, relatives who inherit from and bequeath to each other: a father with his sons, sons with their father, and paternal brothers.
TOSFOS (DH ha'Av) asks the obvious question. Since the type of relationship the Mishnah is discussing is relatives who inherit from and bequeath to each other, why does the Mishnah need to mention the two examples of "a father with his sons, and sons with their father"? Both cases are examples of the same relationship!
(a) TOSFOS answers that the second statement of "sons with their father" alludes to the Halachah that daughters do not inherit when there are sons. Although the Mishnah soon will state this Halachah explicitly, the Mishnah occasionally hints to certain Halachos tangentially.
(b) The RASHBA also explains that the second case is intended to exclude daughters, but he says that it is necessary for the Mishnah to hint this for the reader of the Mishnah. Had the Mishnah said only "ha'Av Es ha'Bonim" -- "a father with his sons," one would have concluded that not only does a father inherit from his sons and daughters equally, but they also inherit from their father equally. The Rashba points out that there is a Kal va'Chomer which illustrates that a father inherits the property of his daughter: if a father inherits from his son even though the father has no special monetary rights to his son, then certainly a father should inherit from his daughter, since the father has certain monetary rights to his daughter (i.e. Kidushin, Amah Ivriyah). This is why the Mishnah adds the example of "sons with their father," implying that sons inherit their father before daughters.
(c) The TOSFOS CHADASHIM on the Mishnayos quotes the MAHARSHACH who explains that without both cases, one might have thought that if a father dies and is survived by his father and his children, his father -- and not his children -- inherits his property. The Mishnah therefore adds the second case of "sons with their father" to teach that the sons inherit their father even when his father is still alive.
(d) The MELECHES SHLOMO explains that the second example is not superfluous, since it parallels the structure of the Mishnah which begins with the words, "Nochlin u'Manchilin" -- "they inherit and bequeath." It is appropriate to match the double wording of the opening of the Mishnah with the two cases of "a father with his sons, and sons with their father." (Y. MONTROSE)
2) WHY A MAN'S SON IS FIRST TO INHERIT HIM
QUESTIONS: The Beraisa states that when a man dies with no children, his father inherits his property before his brothers. One might have thought that a father inherits his son's property even before the deceased man's son. The verse therefore states that "ha'Karov" -- "the closest [relative]" -- inherits him. (See RASHBAM, DH ha'Karov, who explains why a man's son is considered a closer relative to him than his father.)
The Beraisa then asks an apparently odd question: why did the Chachamim place the son of the deceased before the brother of the deceased? It then gives arguments for both possibilities. A man's son should inherit him because the son takes the place of his father in cases of Yi'ud and Sedeh Achuzah. On the other hand, a man's brother takes his place for Yibum. The Gemara answers that a man's brother takes his place for Yibum only when the man dies with no son. This proves that a man's son is "closer" to him that his brother. The Gemara continues this discussion.
Many Rishonim are bothered by the Beraisa's question. The verse explicitly states, "When a man dies and he has no son, you shall pass his inheritance to his daughter. If he has no daughter, you shall give his inheritance to his brothers" (Bamidbar 27:8-9). Why does the Beraisa need to bring arguments to prove whether or not the inheritance should go to the son? The verse explicitly states that it should go to the son before the brother! (See RASHBA.)
(a) The RAMBAN answers that the style of the Gemara is to ask questions not for the sake of getting an answer, but in order to gain a clearer understanding of the topic.
(b) The RA'AVAD understands that the Gemara is asking a different question. The Gemara's question is why did the Chachamim state that a man's son should inherit him before his father, and entirely remove a brother from inheriting before the son or the father? Although the verses clearly state that a son inherits before a brother, the brother at least should inherit before the father. The Gemara advances an argument to this effect by showing that a brother may even be closer to the deceased man than his son, were it not for the fact that the Torah states that the son inherits first. (Y. MONTROSE)