Her sister Jessica and brother-in-law Nick Lachey witnessed the impromptu jam at the Caesar's Palace, along with the production team for MTV's "Ashlee Simpson Love and Legal," the Las Vegas Review-Journal reported.
After years of passing herself off as her elder sibling, Simpson said she's glad she can not just be herself.
"I used to use my sister's ID, which is really lame. They were like, 'Jessica Simpson ... You're Jessica Simpson?' And then I used to have one that said, 'Luanne,'" she said, Zap2it.com reported.
After an embarrassing lip-synching incident on "Saturday Night Live" last year, Ashlee returns to the scene of the crime this Saturday as the musical guest of Jon "Napolean Dynamite" Heder.