Reference for Pesachim 101:8

ואתו אינהו וגזור בשני אחין משום בעל אחותו תנא תלמיד לא ירחץ עם רבו ואם רבו צריך לו מותר

[That they do not go out] in slippers too [is understandable], lest they fall off and they come to carry them four cubits in the street. But what is the reason that [brothers] do not bathe [together]? - As it was taught: A man may bathe with all, except with his father, his father-in-law, his mother's husband and his sister's husband.<span class="x" onmousemove="('comment',' In their case this may lead to impure thoughts.');">¹³</span> But R'Judah permits [a man to bathe] with his father, on account of his father's honour,<span class="x" onmousemove="('comment',' He can perform some services for him and help him.');">¹⁴</span>

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