Gemara

(Mishnah): A man may be secluded with his mother or daughter.

(R. Yochanan) Question: "If your brother, the son of your mother will entice you (to serve idolatry)" do only maternal brothers entice?!

Answer: Rather, this hints that a man may be secluded with his mother, but not with other Arayos.

81b (Rav Yehudah citing Rav Asi): A man may be secluded with his sister. He may live with his mother or daughter.

(Shmuel): One may not be secluded with any Ervah, or even with an animal.

Question (Mishnah): A male may be secluded with his mother or daughter. He may sleep with them b'Kiruv Basar (without clothes).

Counter-question (Shmuel - Beraisa): A man may be secluded with his sister, mother-in-law, or any Ervah, only if witnesses are there.

Answer to both questions: Tana'im argue about seclusion with relatives.

(Beraisa): R. Meir would not be secluded with his daughter. R. Tarfon would not be secluded with his daughter-in-law. A Talmid mocked R. Tarfon for this. Shortly after this, the Talmid sinned with his mother-in-law.

R. Acha bar Aba visited Rav Chisda (his son-in-law). Rav Acha put Rav Chisda's daughter b'Kanfei (in his lap, or under the sheet, to sleep with her b'Kiruv Basar).

Rav Chisda: She is Mekudeshes! Shmuel taught that we do not use a woman!

R. Acha: I hold like another teaching of Shmuel, who permits what is done l'Shem Shamayim (e.g. to show my daughter that I cherish her daughter).Rishonim

RISHONIM

The Rif and Rosh (Kidushin 32b and 4:21,24) bring R. Yochanan's Drashah, the Mishnah, Rav Yehudah and Shmuel.

Ran (DH Lomar): R. Yochanan expounded that one may be secluded only with his mother. Really, we permit also with one's daughter or sister (81b).

Ran (DH Misyached): One may be secluded with his sister on occasion. They may not dwell alone together in a house. One may dwell with his mother or daughter. The Yetzer Ra does not incite for them, due to the Tefilah of the Great Assembly (Sanhedrin 64a). We follow the Stam Mishnah against the Beraisa.

Rosh: Tosfos rules like Rav Asi, for he was greater than Shmuel. Even though the Gemara brings many Amora'im who were stringent, this was Midas Chasidus. One who is stringent will be blessed.

Rambam (22:1): One may not be secluded with any Ervah, whether she is old or young, for this is leads to Bi'ah. The only exceptions are a mother with her son, a father with his daughter and a man with his wife when she is Nidah.

Rambam (2): The Isur of seclusion with Arayos is a tradition.

Rambam (Hilchos Sotah 1:3): If a man warned his wife against seclusion with Ploni and Almoni (together), and they were secluded, she is forbidden. This is even if they are her brothers, or her father and brother.

Rashba (1188): A Beraisa forbids seclusion with a sister, mother-in-law and the rest of the Arayos.

Tosfos Rid (81b DH Omar): Rashi says that Rav Chisda said that one must distance from a married woman. Why should one need to distance more than from a single granddaughter?! Rather, because she is Mekudeshes she is like an adult, and you should not put her in your lap, like one does with a minor.

Poskim

Shulchan Aruch (22:1): One may not be secluded with any Ervah, old or young, for this leads to Bi'ah. The only exceptions are with a child or Ishto Nidah

Chelkas Mechokek (1): These may live together in seclusion even on a fixed basis. A sister is permitted only occasionally.

Beis Shmuel (1): Tosfos (Shabbos 13a DH Mah) and the Tur say that the Isur of seclusion is mid'Oraisa. The Rambam says that it is a tradition, for there is only a hint from the Torah, so one is lashed only Mardus (mid'Rabanan). Chayavei Lavin are included in Arayos. The Rosh says that the Halachah follows Rav Asi. Why did the Rambam and Tur omit this? Perhaps they hold that the Halachah follows him against Shmuel, but not what he added to the Mishnah. The Mishnah permits only with one's mother.

Question (Mishneh l'Melech Hilchos Sotah 1:3 DH Kina): The Yerushalmi says that the opinion that says that one can warn his wife against seclusion with her father or son, even though the Torah permits seclusion with them, holds that he can warn against seclusion with two men. Perhaps the Torah permits only a Penuyah to be secluded with her father or son! Rav Chisda rebuked Rav Acha for not distancing from an Arusah!

Answer (Mishneh l'Melech): Rav Chisda rebuked Rav Acha for putting her in his lap, which is like hugging and kissing. The Rambam (Hilchos Isurei Bi'ah 21:6,7): permits hugging one's daughter, but not if she is Nesu'ah. He holds, akin to the Tosfos Rid, that she is like an adult only if she is Nesu'ah. All agree that there is no Isur of seclusion with a mother or daughter. The Rambam mentions Kinuy from brothers, even though the Mishnah did not discuss this, because Rav Asi permitted seclusion with one's sister. Why didn't the Rishonim bring his law? Shmuel argues only because he rules like R. Meir! The Rosh explained why the Halachah follows Rav Asi! The Beis Shmuel says that we do not rule like Rav Asi's addition to the Mishnah. This is wrong. If so, why did the Rambam say that Kinuy may be even from her brothers? He should have taught that it is even from her sons! It is difficult to say that he merely adopts the Yerushalmi's text, which holds like Rav Asi.