The Rambam in ????? ? ? does not count marrying two sisters as one of the arayos that a ben noach is assur to marry.
According to the tzad that the avos had the dinn of jews, it does seem that Yaacov’s marriage to Rochel was achos ishto, which I think would make Yosef a mamzer. (this may help to understand why his brothers didn’t respect him)