Gemara

114b: The Mishnah teaches that a mother bequeaths to her son, and also that a son inherits his mother! The repetition equates a son to a husband. Just like a dead man does not inherit his wife (to pass her property to his heirs), also a dead son does not inherit his mother to bequeath to his paternal brothers.

(R. Yochanan citing R. Yehudah b'Rebbi Shimon): "Matos" equates the mother's and father's tribes. Just like a father inherits his son, also a mother.

158b (Chachamim of Eretz Yisrael): If Reuven borrowed from his father (Yakov) in Yakov's lifetime, and he died; Reuven takes from the buyers. This is among the most difficult monetary laws.

Objections: Why does borrowing entitle him to take from buyers? Also, the teaching did not mention that anyone bought anything!

Correction: Rather, Reuven sold Yakov's property in Yakov's lifetime, and Reuven died. After Yakov dies, Reuven's son takes from the buyers. This is difficult. Why can't the buyers say 'you do not inherit what your father sold!'

Objection: This is not difficult! His son can say 'I do not take due to Reuven. Rather, I inherit Yakov - "Tachas Avosecha Yihyu Vanecha."'

Answer: "Tachas Avosecha... " does not teach that one directly inherits his grandfather. It is just a blessing (you will have children to inherit you).

Question: It is not just a blessing. The law is true!

(Mishnah): A house fell on Reuven and his father Yakov or Morishav (someone else whom Reuven inherits). Reuven left creditors. The creditors say that Yakov died first (and Reuven inherited him, so they collect from the Yakov's property). Yakov's heirs say that Reuven died first.

Suggestion: Yakov's heirs are Reuven's sons. 'Morishav' is Reuven's brother. If a grandchild does not directly inherit his grandfather, the verse is merely a blessing. Even if Reuven died first, the creditors should collect, for the property passes to Reuven before it goes to Reuven's sons!

Answer: No, Yakov's heirs are Reuven's brothers. 'Morishav' is Reuven's uncle. (However, if Yakov's heirs were Reuven's sons, the creditors would collect no matter who died first.)

Question: If a woman died after her son, does he 'inherit' her in the grave, i.e. to pass her property to his paternal brothers?

Answer (Rav Acha bar Minyomi - Mishnah): If a house fell on Reuven and his mother, his heirs divide her property with her heirs.

If a son inherits in the grave, even if the son died first, he inherits his mother to pass her property to his paternal brothers!

(Abaye): A son does not inherit in the grave, because the Torah mentions transfer of inheritance (to another tribe) regarding a son and a husband who inherit). Just like a husband in the grave does not inherit his wife, also a son.

Bava Kama 108b (Mishnah): If Reuven stole from his father, swore falsely to him, and his father died, he pays to his children or brothers.

Rishonim

Rif and Rosh (8:6): A son does not inherit his mother in the grave

Rambam (Hilchos Nachalos 1:13): A son does not inherit his mother in the grave, i.e. to pass her property to his paternal brothers. If Reuven died, and afterwards his mother died, we do not say that since if he were alive, he would inherit her before anyone else, also now his heirs inherit her before anyone else. If so, his paternal brothers would inherit her. Rather, if her son has descendants, they inherit her. If not, her inheritance goes to her father's family.

Magid Mishneh: Likewise, he does not inherit to bequeath to his father, e.g. if his mother was divorced. The Rashba says so; this is clear.

Hagahos Ashri (9:38): The Gemara concludes that "Tachas Avosecha" is a Berachah, so the Halachah is difficult, but it is the Halachah. The Rashbam explained regarding a sale, but the same applies to a lien. If property fell to Reuven after his death from his mother's father, his paternal brother can inherit only due to Reuven, therefore Reuven's creditor collects from the property.

Tosfos (Bava Kama 108b DH l'Vanav): If Reuven stole from his father Yakov and swore falsely, and Yakov died, Reuven pays (the share that he would have inherited) to his own children or brothers (if he has no children). We view it as if Reuven died, and his children inherit Yakov with Reuven's brothers. They inherit due to Yakov, not due to Reuven, like it says in Bava Basra "Tachas Avosecha Yihyu Vanecha."

Chazon Ish (ibid.): This shows that we conclude that "Tachas Avosecha..." is not merely a Berachah, and a grandchild has his own Zechus to inherit. He does not merely inherit from his father (for here, Reuven may not take the property). However, one could say that Reuven bequeathed to his children the Zechus to inherit.

Tosfos (114b DH Af): R. Yehudah b'Rebbi Shimon teaches that a convert inherits her son (when there are no other heirs).

Poskim

Shulchan Aruch (CM 104:16): If David borrowed and had no money to pay, and a house fell on him and Morishav, and his children say 'our father died first, and inherited in the grave. The creditor does not collect' (Rema - for a creditor does not collect from Ra'uy), and the creditor says 'Morishav died first, and I may collect from the property', the Halachah follows the heirs.

Question (Pischei Teshuvah 14): Hagahos Ashri says that If property fell to Reuven after his death from his mother's father, his paternal brother can inherit only due to Reuven, so Reuven's creditor collects. A dead son does not inherit his mother to bequeath to his paternal brothers!

Answer #1 (Dagul me'Revavah): The case is, his mother is a convert without heirs. Therefore, her son inherits her in the grave to bequeath to his paternal brothers. If not, the property would be Hefker. We find such a distinction: normally, a mother does not inherit her son, but a convert inherits her son (Tosfos). The Shach (21, and 280:3) rules like Hagahos Ashri.

Answer #2 (Tumim 14): The Maharil (2) says that Maharam says that if Yakov died and there was a lien on his property to pay his wife's Kesuvah, and afterwards his son Levi died and the lien was lifted, Levi's wife does not collect from Yakov's property. This is because in Levi's life it was only Ra'uy, due to the lien. However, in Shevuos, Maharam ruled that if Reuven died and left a daughter, and she died and the lien to pay a Kesuvah to Reuven's wife was lifted, his daughter's husband inherits, even though a husband does not inherit Ra'uy! Also the Rosh says that a Bechor inherits double from property on which there was a lien to pay a Kesuvah. The Maharil (74) says that some things are considered Muchzak regarding inheritance, which is automatic, but for a creditor, who seeks to take property, they are considered Ra'uy. This answers the questions. Hagahos Ashri discusses when there was a Kesuvah against the grandfather's property. It is a Zechus for an heir to consider the property Muchzak, and he can opt to consider it Ra'uy. E.g. one can say 'my grandfather's property was Ra'uy, so my father's creditor has no claim on it. I inherit my grandfather.' This is why he discussed a paternal brother, who can inherit only from his dead brother.

Nesivos ha'Mishpat (1): All agree that a son inherits Ra'uy! Rather, the Ketzos ha'Choshen (15) explains that the claim 'I inherit my grandfather' is not enough, for the creditor can say 'your father inherited first, so I have a lien due to him.' Therefore, we must also say that it is Ra'uy, and a creditor does not collect from Ra'uy. 'I inherit my grandfather' dispels the Mitzvah to pay one's father's debts (from what he inherited from his father).

Answer #3 (Chazon Ish Bava Kama 15:8): Paying a creditor is as if the son himself consumes the money. Therefore, a son can inherit his mother in the grave to pay his creditor! Indeed, if the Halachah were that a Mes never inherits in the grave to pay his creditor, rather, the living inherits the last one to die, one would not inherit in the grave to pay his creditor. However, the Gemara says that the Mes should inherit in the grave to pay his creditor, just the heir can claim 'I inherit my grandfather; the Mes never inherited.' Here, Reuven's brothers can inherit only through him, therefore, the creditor collects. Even though there is no lien on the property, and it is a mere Mitzvah mid'Rabanan for the heirs to pay, Chachamim consider the property to belong to the Mes when the heirs can inherit only through the Mes. If a convert's son died, and then his father died, the son inherits him for the son's creditor to collect from, even if the son has no heirs. Chachamim obligated orphans to pay their father's debt, lest he fulfill "a Rasha borrows and does not pay."

Question (R. Akiva Eiger 138): Sefer ha'Terumos says that if a Shechiv Mera gave property to one of his nephews, we do not say that the nephew comes from his grandfather's rights. Why did the Poskim omit this?

Answer (Pischei Teshuvah 14): The Shulchan Aruch holds that a creditor does not collect from Ra'uy, so in either case he does not collect. However, according to Sefer ha'Terumos the nephew is obligated to pay the debt of his father. He cannot say 'I directly inherit my uncle.'

Shulchan Aruch (CM 276:5): A son does not inherit his mother in the grave, to pass her property to his paternal brothers. If Reuven died, and later his mother died, we do not say that since if he were alive, he would inherit her, also now if he has no descendants, his paternal brothers inherit her. Rather, her inheritance goes to her father's family, since her son has no descendants.