I think that Sedaris has his family's tacit or even explicit permission to use their real names. They seem to accept the fact that he does so.

If you're writing a memoir, whether it's okay to use real names probably depends in part on who you're writing about and what you have to say about them.

Again, they may be unhappy about what you have to say but if what you have to say is true they won't have much of a legal leg to stand on.

But if you write a memoir which defames somebody and you name them that could cause problems.

If a traditional publisher is publishing that memoir, their legal department is going to take a look at it and make sure it isn't going to open them up to any liability.

And most publishing contracts contain clauses in which the writer is required to state that nothing in the book is libelous or defamatory -- and must agree to hold the publisher harmless if there's a successful lawsuit.