May siblings (boys and girls) go swimming together in a private pool?
If the boy is under age 9 and the girl is under age 3, according to all opinions, they may swim together. However, some hold that if the girl is over age 3 and the boy is 9 and up, they may not swim together. Rav Mordechai Eliyahu Shlita (Former Sefardi Chief Rabbi of Israel), gave a psak based on the Chazon Ish, that even if the girl is over age 3 and not physically developed, she may swim with her brother.(See Shulchan Aruch, Even Ha'ezer 22:11, and "Kedoshim T'hiyu" by Rav Shmuel Katz, pg. 65)