MAZAL TOV ON SEDER NASHIM!
R' Moshe (E.H. 4:64:1) was asked whether a family who converts have an issur yichud since halachically they are no longer related - גר שנתגייר כקטן שנולד דמי. His basic approach is that the heter for yichud is not based on the halacha of being related, rather on the metzius of whether or not there is desire for these relatives. Being that both jews and goyim don't have a yetzer hara for blood relatives, he paskens that there isn't any issur of yichud.
In the course of the discussion, R' Moshe raises a tremendous question. Rashi explains that the reason that there is no yetzer hara for a mother and daughter is because the anshei k'neses ha'gedola davened to remove the yezter horah for blood relatives. However, the gemara on 80b learns out from a pasuk that yichud with a mother. Clearly, even prior to the anshei k'neses ha'gedola there was a heter of yichud with one's mother, so the heter can't be based on their tefillah to remove the yetzer hora? Based on this question R' Moshe explains that between a parent and child the nature from the time of creation is that there is no yetzer horah. That explains why Lot's daughters had to give him to drink wine in order to sleep with them, since he had no desire for them. Rashi is citing the gemara in sanhedrin where they davened to remove the yetzer horah for arayos, to explain why there is no issur yichud by a sister. This is not part of the nature of the human being, and was only a result of their tefillah. Included in the tefillah of anshei k'neses hagedola is also the sister of one's parent, where prior to their tefillah, there was a desire for them. Nevertheless, R' Moshe assumes that even the removal of the yezter horah for sisters and aunts, applies to goyim just as it applies to jews.