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A "Brain Sister" is a girl who wears glasses and finished high school. However, there is some debate as to whether or not a "Brain Sister" actually wears glasses.
"She was only twentyseventeen, she was a lonely beauty queen, she was a brain sister, Texas Instruments all day..."

Quoted from the Limozeen song "Brain Sister"
by Garboth March 19, 2008
An attractive twenty-something year old who is constantly occupied with a Texas Instruments calculator.
"She was a brain sister - texas instruments all day"
-"Brain Sister" by Limozeen
by Daniel C. Johnston July 31, 2006