Question

Hello Rabbi,
Recently I studied a bit of the laws of seclusion. It is clear to me from the Talmudic passage that the prohibition is because of a concern about sexual relations (and not, as someone once suggested to me, that there is something inherently problematic about the very fact that a man and a woman are alone together in the same place). Rav Assi, on page 81b in tractate Kiddushin, adds to the Mishnah that a person may be secluded with his sister. Rav Assi does not seem to have a tannaitic source, so the only way to explain his view is that he says this on the basis of reasoning. The exposition of the verse, “If your brother, the son of your mother, entices you,” teaches that a person may be secluded with his mother. The Mishnah adds that he may also be secluded with his daughter, and it is clear that it derives the reason from the verse—just as a person does not desire his mother, so too he does not desire his daughter. Rav Assi apparently uses the Mishnah’s rationale and adds that he also does not desire his sister.
The Talmud there brings Rabbi Meir’s statement, in which he says, “Beware of me because of my daughter.” I find this saying very difficult. In normal cases a person is not supposed to be attracted to his daughter. (I would note that if Rabbi Meir was in fact attracted to his daughter, and he told his household members to help him, that would testify to his greatness, and if only all rabbis and great figures would learn from him.) In my humble opinion, the very question is inappropriate, at least in normal cases. I can imagine a situation in which I might fail with another man’s wife, even though I know it is forbidden, and although inside I feel that it really would not happen, even if I were secluded with her. I agree that “there is no guardian against sexual prohibitions,” and I am not willing to rely on my gut feeling in that case. But when it comes to my mother, my sister, or my daughter, the question does not even begin. The very question disgusts me.
I feel the same way regarding seclusion with my sister-in-law and with my mother-in-law. My wife’s family is observant of Torah and commandments, (but?) they are not strict about the laws of seclusion and physical contact with more distant relatives—cousins, uncles, etc. More than once I have been in seclusion with my mother-in-law. Should I avoid such situations? I think that even bringing it up is immodest, because there is not even the slightest initial thought that anything could happen between us. If I had not known this Jewish law, the thought would never even have occurred to me, and if I tried to explain this to a secular friend, I think he would call me a pervert—and rightly so.
 
Recently I heard a series of your lectures about autonomy in Jewish law. Is this a case where I can derive the Mishnah’s rationale and extend Rav Assi’s view to permit seclusion with my sister-in-law and mother-in-law (not on a regular basis, but occasionally)? Aside from the Rosh, it does not seem to me that most of the medieval authorities even brought Rav Assi’s view as practical Jewish law, but clearly today everyone is secluded with his sister, and I definitely do not agree that in this case “blessing will come upon one who is stringent.” Can Jewish custom, or in this case the custom of my wife’s family, determine the Jewish law in this case?
 
Thank you very much,