When Sam runs around the end of the bar to go to Rebecca's office to try to postpone their date, he grabs the end of the bar as he swings around and the whole bar can seen to move several inches, showing that the bar isn't actually a permanent fixture.
Sam says he didn't know Rebecca had an actress sister. Sam obviously has a poor memory; Rebecca told him so in the Season 6 episode Paint Your Office (1987). This is only partially true. In "Paint Your Office" she said she had a sister but nothing about horror movies.