Regarding the subject of Yibum (Levirate marriage), what is the source, if any, that this is also an obligation on a Noahide?
Many of the discussions on this Q&A site concerning Yibum connect to the discussion of Yehuda and Tamar. But the Lubavitcher Rebbe citing the Rogatchover Gaon in connection with the subject of the laws associated with a "Rebellious City" makes clear that the Avot were not obligated in the 613 commandments, meaning they were not commanded to observe them. Their fulfillment of the 613 commandments of the Torah was voluntary.
One of the consequences of this distinction which the Rebbe explains was that their observing of the 613 commandments could not be in conflict with those prohibitory laws which were obligatory upon a Noahide at that time.
The prohibitions relating to marriages of consanguinity are generally obligations that do fall upon a Noahide in the sources that I have seen. But since Yibum appears to override that prohibition, I am speculating that perhaps Yibum is also obligatory upon a Noahide.
Please explain any reasoning carefully and cite all sources clearly and completely in any responses posted. And thanks to all for your input.