An intellectual, well-read woman was once known as a bluestocking. You can describe your scholarly sister, who's knowledgeable about many subjects, as a bluestocking — no matter what color her socks are.
The term bluestocking was first used in the 18th century to refer to a group of English women who decided they'd rather have intellectual conversations with educated guests than sit around playing cards and chatting. Because at least one of their guests wore inexpensive, blue worsted stockings, people who disapproved started calling these ladies bluestockings. They quickly reclaimed the term, happily calling their group "The Blue Stockings Society."