Nah, jealous little sister type is my guess.
First, she's jealous her big sis is paying less attention to her, then she starts wanting the guy as well.
Cliche, but still.
Ooooh! We live in the Highlands with our kilts in the wind! We're the Tartan Army, come join us, my friend! We look so dandy, jolly, and bloody right! We wear tartan because of our Tartan Pride!