סימן עב סעיף יג

אָסוּר לִרְחוֹץ עִם אָבִיו וְחָמִיו וּבַעַל אִמּוֹ וּבַעַל אֲחוֹתוֹ. וּבְמָקוֹם שֶׁנּוֹהֲגִין לְכַסּוֹת עֶרְוַתן בְּבֵית הַמֶּרְחָץ, מֻתָּר. וְכֵן הַתַּלְמִיד לֹא יִרְחַץ עִם רַבּוֹ, וְאִם צָרִיךְ לוֹ שֶׁיְשַׁמְּשֶׁנוּ, מֻתָּר.

It is forbidden to bathe⁷⁸Maseches Pesachim 51a. with your father, father-in-law, your mother's husband, or your sister's husband. And in places where it is customary to cover their nakedness in the bath house, it is permitted.⁷⁹Rema [Even Ha’ezer 23: 6]. And also the student should not bathe with his (teacher) Rebbe,⁸⁰If the student was there first, he need not leave if his Rebbe comes, but regarding your father, father-in-law etc. you must leave. See Beis Shemuel [Even Ha’ezer 23: 5]. but if he needs him to assist him, it is permitted.⁸¹This does not apply to your father and the others mentioned above. Regarding them, it is prohibited, even if they need your services.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)