01/17/2012 05:49 am ET

'Late Show With David Letterman': Elizabeth Banks Shows Pic Of Pregnant Sister (VIDEO)

Most pregnant women feel a little sensitive about their size and appearance at some stage, and Elizabeth Banks' pregnant sister Sarah may be feeling just a little more sensitive than most this morning.

The star of upcoming pregnancy-themed movie "What To Expect When You're Expecting," showed "The Late Show With David Letterman" (Weeknights, 11:35 p.m. EST on CBS) a photo of her and Sarah wearing wetsuits, only for host David Letterman to poke fun at her sister.

Banks was describing how she and Sarah had been swimming with whale sharks -- a.k.a. "the largest fish in the ocean" according to her -- when Letterman just couldn't help himself. He quipped, "Are they bigger than your sister?"

Uh oh. Pregnant ladies offended? Check. Banks in trouble with her sister? Check. Letterman did laughingly apologize, and Banks did try to chastise him, but that may not be enough to save her from some sisterly wrath.

"The Late Show With David Letterman" airs Weeknights, 11:35 p.m. EST on CBS.

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