Dash: Check Rambam Melachim where he talks about it. Only a sister from one parent (I think the mother, not the father) is Assur to B’nei Noach. It’s the 6th Halachah in one of the Perakim, I forget which one though. Maybe 9:6?
I’m just curious, by the way, where did you hear that Yosef thought they were B’nei Noach while the brothers thought they were B’nei Yisrael? I thought that was my Chiddush.