Frankie Carmine

When a person dates or marries their sibling's ex. Usually used with the term "pulled a."
Nota Bene: The sibling is the one who pulls a Frankie Carmine, not the ex.
Other terms:
Triple Frankie Carmine: When a third sibling dates or marries the same ex.
Frankgay Carmine: When one sibling is male and the other is female.

Brother 1: Hey, your new girlfriend is pretty hot!
Brother 2: Don't pull a Frankie Carmine on me!