This article is from the The Terminator FAQ, by Karsten A. Loepelmann firstname.lastname@example.org with numerous contributions by others.
Yes, Linda's sister's name is Leslie Hamilton Gearren; she is a nurse in New
Jersey. She was in the scene at the end where the T-1000 took the form of
Connor's mother. Linda actually played the T-1000 version of herself and her
sister played Sarah Connor coming up behind the T-1000. Linda's sister also
appeared in the scene in which Sarah replaces the Terminator's chip and sees
herself in a mirror (this scene is restored in the Special Edition). Linda
Hamilton played Sarah on the playground during her dream sequence (in fact,
she is holding her real-life son).
(The guard in the mental institute also has a real-life twin brother, who
actually played the T-1000 coming up behind him at the coffee machine. These
twin brothers were also in "Good Morning Vietnam" and "Gremlins 2: The New