I don't know where you got that from, that's not a rule. Queen Nis' is also correct. See Merriam-Webster ;)
My English class. I was told the only exceptions are for historical figures, as Merriam-Webster confirms in the classical figures part. Perdue Owl backs me up on that front (see here). I guess it's really just up to interpretation based on your source though, so fair play with Merriam-Webster. Keep up the good work with the writing, boss!