"Sister cars" can be two cars built on the dame day, or very close in VINs, and/or ordered for the same dealership on or about the same time.
My car, and Mike Jewell's CS are sister cars, both GT390, 4spd's ordered and sold at the same time at Suburban Ford in Sacto in April '68.
Hovever, I'm wondering. What would a "sister car" be, if it was also a "bad boy"? (a "Twisted Sister"?)
LOL.
Paul N.