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Gwyneth Philbin is the sister of Darryl Philbin who appears in Season 6 episode The Meeting.

Dwight and Toby go to Darryl's house to investigate whether or not he's really on crutches and unable to work.  When they see Darryl's sister walking around, they assume it is Darryl and Toby calls "him" (really her) an asshole.  Gwyneth turns around and they realize that it is actually Darryl's sister, who closely resembles him. 

She and Darryl confront Toby and Dwight at the office where Darryl exclaims that he is filing a complaint against the pair.