Christine Ebersole

Samantha Who's mother, Regina, has a secret sister —  and she's being played by Tony-winning actress Christine Ebesole, TVGuide.com has learned exclusively.

Whoa, what, Who? A "secret sister"? Well, it's complicated. It turns out that Regina (played by Jean Smart) used daughter Sam's amnesia as an opportunity to "forget" about her sibling's existence. But one unexpected run-in between Regina, Sam and "Aunt Amy" quickly (and comedically) thrusts the truth to the fore.

Ebersole will guest-star in the 14th episode of the ABC comedy's second season. The actress' credits include a pair of Tony Award wins — for Grey Gardens (in 2007) and 42nd Street (in 2001) — and the TV series Related.