HaLeiVi: Why would I advise someone against marrying a niece? Birth defects? That’s a bit of an issue but not so much. I could hear the Tayneh that the reason for marrying a niece doesn’t apply so much anymore, but I don’t see any inherent problem with it.
And I’m not confusing anything. It’s an Aggadta in Brachos that says how great a city was that had 80 pairs of Kohanim marry 80 sisters and that the searched and the only thing they found like it was that a certain Amora and his brother had married 2 sisters.