I agree with you. It seems odd given the timeline and location of her disappearance, her age, her height, weight, hair and style, the tattoo on her right shoulder vs a vicious bite mark on WCJD's right shoulder, overall general appearance that she was not ruled out long ago. Could the Chevrolet have been the same car? Some reports list Theresa's car as gray others as green.
Girls who look even less like WCJD, were well into their 20s, and who were not from Texas were considered, so I think you have a validpoint.
If her sister is still around, it would be helpful to know more about the diastema. In the one clear photo it is definitely visible and does seem large enough to be notable. It is possible she wore a retainer and the gap closed. Knowing when that photo was taken in relation to her disappearance would be VERY helpful, IMO. It looks like it was cropped from a family or other group image to me.