Is it permissible to hear my sister sing? Is it permissible to touch her?
It is customary in many places to permit Kol Isha for sisters.
Although some equate a sister with other girls, and some write that it is praiseworthy to be stringent, one may certainly follow the most lenient opinion and permit the hearing of a sister’s singing up to 11 years old.
It should be added that the general custom, in particular with regard to the singing of Zemiros on Shabbos, is to be entirely lenient with regard to Kol Isha of sisters (see Avnei Yashfei Vol II, 5:8, and Be’er Sarim Vol I, 30:4, who are generally stringent concerning the singing of sisters over the age of 11, but is lenient concerning singing zemiros together — provided the girls sing in a gentle voice).
It is permitted to touch a sister under the age of 11; above that age, one should refrain from this, though it is not a full prohibition.
Concerning the age from which kol isha is prohibited, see Shulchan Shlomo, 75:3; Salmas Chaim 93; Rivevos Efraim 7:p. 246; Ishei Yisrael 55:note 99 — which present the more stringent opinion (3).
The Chazon Ish, Orach Chaim 16:8 maintains that there is no set age, and everything depends on the girl looking like a mature girl. Ben Ish Chai (Bo 1:13), Oz Vehadar Levushah (p. 47), and Laws of Daily Living (1:pp. 84-85) give the age as 6, whereas Halichos Bas Yisrael (6:note 120) and Halichos Shlomo (Tefilla 20) mention the age of 9. Iggros Moshe (1:26) writes that in pressing situations one may be lenient up the age of 11.
Concerning the status of sisters, a number of authorities do not make any distinction between sisters and other girls (see Be’er Sarim 1:30:4; Halichos Bas Yisrael (p. 85) citing from Chazon Ish; Divrei Chachamim (p. 253) citing from Harav Yaakov Kamenetsky; Rivevos Efraim (vol. 7, p. 246); Orchos Rabbeinu (vol. 4, p. 156); among others).
However, Avnei Yashfei (2:5:8) quotes from Harav Elyashiv that with regard to sisters one may be lenient up to the age of 11, and a number of authorities quote the general custom to be lenient with regard to sisters (see Divrei Chachamim p. 253 quoting from Harav Moshe Feinstein; Hakatan Vehilshosavchap. 81, note 33; Taharas Am Yisrael p. 53.
The questioner in Be’er Sarim(1:30:4), Rabbi Wagshal of Gateshead, quotes that the custom in Brisk was to be lenient with regard to Kol Ishah of sisters. However, as these authorities themselves point out, it is preferable to be stringent. Yet, there is certainly leniency in Zemiros etc. where the family sings together.
Concerning hugging and kissing sisters see Rambam (Issurei Biah 21:6) and Shulchan Aruch (Even HaEzer 21:7); for the age of 11 see Shulchan Aruch 73:4.
2. E.H. 21:7.