This article is an excerpt from our Sefer
May a couple have intimacy in the home of their in-laws?
Yes. A couple who is staying as a guest in the home of his or her parents may engage in intimacy, so long as they have their own room and use their own sheets or towels, as stated above. See next!
Living in the home of one’s in-laws:
The Sages prohibited a man from living in the home of his in-laws due to it being considered a brazen act. Nonetheless, if the couple has been designated their own room, as stated above, it is permitted for them to live there [and be intimate]. Some Poskim learn that the prohibition only applies if the mother in-law lives at home, due to worry of a forbidden relationship. Other Poskim, however, rule the prohibition applies even in a case that the mother in-law does not live at home. Practically, the widespread custom today is to be lenient in this matter [and live in the house of one’s in-laws, even if the mother in-law is there, and to permit having intimacy while there]. [This discussion only applies to the home of the wife’s parents, however, to live in the home of the husband’s parents is permitted without restriction according to all. Nevertheless, intimacy would only be allowed if they have their own designated room, as stated above. However, seemingly it is not necessary for them to have their own sheets and towels if they are staying there for a while and considered part of their parents’ household.]
 See S’dei Chemed Mareches Ches Kelal 141; Shevet Halevi 4:166
 Rambam Issurei Biyah 21:15; Kiddushin 12b, “Rav would give lashes to a man who lived in his in-laws home”; Bava Basra 98b; Bei Yosef E.H. 26:4; Kitzur SHU”A 152:12
The reason: As a) One may come to sin with his mother in-law. [Rashi Pesachim 113, brought in Tosafos Riy Hazakein Kiddushin ibid] or b) As its not Tzenius, just as one should not bathe with his father in-law.
Other opinions: Some Poskim rule that this is not a true prohibition, but rather a matter of Derech Eretz. [Neharda in Kiddushin ibid; Tosafos Kiddushin ibid; Rosh Kiddushin 18; See Beis Yosef ibid] Some Poskim rule that this prohibition only refers to a Chasan who is engaged to be married, and is not yet married. [Tosafos Riy Hazakein Kiddushin ibid; China Vechisda 2 p. 220]
 Ra’avad on Rambam ibid; Kitzur SHU”A ibid
 Rashi Pesachim 113
 Tosafos Riy Hazakein Kiddushin 12b
 Tosafos Kiddushin ibid; Rosh Kiddushin 18; Meiri; Shevet Halevi ibid; See Beis Yosef ibid [this law is omitted from the Shulchan Aruch]; See Bava Kama 112a that Rav Yirmiyah lived with his in-laws
The reason: a) It is done for Tovas Hana’ah and due to lack of living space, b) there is really no prohibition to begin with, as it’s merely a matter of Derech Eretz, c) as we hold that the prohibition only applies prior to the wedding, or d) the prohibition only applied in previous generations, however, today Benos Yisrael are more careful. [See Poskim ibid; Shevet Halevi ibid]