English teachers the world over seem to teach different things. Who knows? Maybe both are correct. I don't have an English grammar text book in my possession. I'll bet most native English speakers don't. I've been taught that regardless of singular or plural, you do not append an additional 's' to the end. So I've always considered it Lois'. I also think Lois's just looks plain wrong.
But hey, I don't have the documentation to prove it.
