Well, the original storyline in the first movie was about Elastic-girl's first husband being resurrected as an evil super villain. ANd later he would recruit his daughter Violet to join him in a sort of darth vader twist with Mr Incredible playing the role of the well meaning stepfather who trying to violet back to the side of good.
The idea went nowhere very fast so they switched gears and made Mr Incredible's past be the villain instead, forcing them to drop the stepparent angle completely, but they still liked Violet's design, thus it stayed
