Continuity mistake: When Stewie is trying to get a drink from Brian, there are two alcohol bottles on top of the table. But one is gone for two shots, then comes back. Also the bottles change from closer to father.
Continuity mistake: Brian insults Stewie about the water and there is a water glass by Bryan's plate. Then it cuts to a shot of Stewie and Chris, and the glass is gone.
Chosen answer: It's a real song called "Elvira". Originally by Dallas Fraizer in '66, however, this version seems to be the Oak Ridge Boys cover version. However, the mega lesbians skip the verse and just sing the chorus after the opening line.
Bishop73