Yichud between a brother and sister:[1]

Some Poskim[2] rule it is permitted for a brother and sister to remain alone together in the same room on a temporary basis.[3] [This applies even to a maternal or paternal sister.[4]] Other Poskim[5] rule it is forbidden for a brother and sister to remain together in Yichud even temporarily. Practically, the main ruling follows the lenient opinion and so is the custom of all Jewry.[6] Nevertheless, one who is stringent to not have Yichud with a sibling of opposite gender is blessed.[7] This especially applies in a promiscuous generation [and to children who are going through puberty during their pubescent and prepubescent years[8]].[9] [Some Poskim[10], however, rule that a sibling who is known to be promiscuous may not share even temporary Yichud with a sibling if opposite gender.] According to all opinions, it is Rabbinically[11] forbidden to have Yichud with a sibling of opposite gender on a permanent basis.[12] The Poskim debate the exact definition of permanent in this regard, as explained next.

______________________________________________________________________

[1] See Nitei Gavriel Yichud 3; The Laws of Yichud [Dubov] p. 9

[2] Rav Assi in Kiddushin 81b “A man may have Yichud with his sister and live with his mother”; Yerushalmi Sotah 1b; Chelkas Mechokei E.H. 20:9, 21:1; Beis Shmuel 22:1 [unlike 20:14]; Rashal in Yam Shel Shomo Kiddushin 27; Perisha 22:3; Kitzur Piskeiy Rosh Kiddushin 24; Semak 99; Ran Kiddushin ibid; Mishneh Limelech Sotah 1; Chochmas Adam 126:12; Chikrei Lev E.H. 6 and 17; Avnei Nezer 2:233; Aruch Hashulchan 22:2; Maharsham Mafteichos 76; Nidchei Yisrael [of Chofetz Chaim] 24:6; Nachalas David 23; Erech Shaiy E.H. 22 based on Zohar; Misgeres Hashulchan 152:1

[3] The reason: Although a sister is an Erva, nonetheless, the Sages [Yuma 69b; Sanhedrin 64] removed the inclination for relatives such as one’s sister, and hence permitted temporarily living together. [Tosafus Harosh Kiddushin ibid; Hamakneh, Rashash and Pnei Yehoshua Kiddushin ibid; See Eiyn Yitzchak E.H. 8 and Nitei Gavriel 3 footnote 1 and 2]

[4] Dvar Halacha Hosafos 2; Igros Moshe 4:64-1; Kinyan Torah 5:126; Nitei Gavriel 3:11 footnote 21; See however Toras Hayichud in name of Rav SZ”A

[5] Beis Shmuel E.H. 20:14 “And so too Yichud is forbidden” [unlike 22:1]; Radbaz 7:32; Yosef Ometz 22; Od Yosef Chaiy Shoftim 3 “Although there are Poskim who permit Yichud with a sister, the main opinion follows that it is forbidden, especially in this promiscuous generation.”; Shmuel in Kiddushin 81b [regarding all Arayos, including mother]; Implication of Rambam, Tur and Michaber who all omit the allowance of Yichud with a sister even though they mention the allowance of a mother [so leans to understand Beis Shmuel ibid; See Eiyn Yitzchak E.H. 8 and Nitei Gavriel 3 footnote 1 for an analysis on their opinion]; Implication of Kitzur SHU”A 152:1 who omits a sister

[6] The vast majority of Poskim [brought in footnote above] all rule it is permitted and that so is the main opinion, and so is the custom; Nitei Gavriel 3:1

[7] Chelkas Mechokei E.H. 21:1; Piskeiy Harosh; Semag L.S. 126; Rashal ibid; Chochmas Adam ibid; Misgeres Hashulchan ibid

[8] Heard from a child psychologist with decades of experience in dealing with incense and molestation cases that occur within the family

[9] Poskim brought in Nitei Gavriel 3 footnote 4

[10] See Nitei Gavriel 3:12; The Laws of Yichud [Dubov] p. 10 footnote 25

[11] Is this prohibition Biblical or Rabbinical? This prohibition is merely Rabbinical. [See Beis Shmuel ibid; Piskeiy Harosh 4:22; Nitei Gavriel 3 footnote 2]

[12] All Poskim ibid; Yerushalmi ibid; Rashi Kiddushin ibid in explanation of opinion of Rav Assi

The reason: The inclination for a mother or daughter is completely dormant and Yichud is hence allowed even permanently. However, the Yetzer for a sister although dormant is still possible, and hence the Sages limited the Yichud allowance to only a temporary basis. [See Rashi Kiddushin ibid; Pnei Yehoshua ibid; Tosafus Rosh Kiddushin ibid] Alternatively, it is not possible to decree against living alone with a mother or daughter, as opposed to a sister. [Erech Shaiy ibid]

[13] Heard from a child psychologist with decades of experience in dealing with incense and molestation cases that occur within the family