Ashley Benson who plays Barney's sister is the co-star of Lucy Hale on Pretty Little Liars, who also plays Robin's sister
This episode put an end to a popular fan theory that Barney's sister would be The Mother.
When Ted says "Charlie was everywhere" when talking about the Vietnam War, this is a reference to the communist forces in Vietnam. EDIT: to be exact, "Charlie" is short for "Victor Charlie", which refers to the US military phonetic spelling for VC, which is an abbreviation of Viet cong, which is short for Viet Nam cong san - which translates to "Vietnamese communist(s)".