The only thing in print to point to a prior relationship is the Rick Hendrick interview which you can read here :
Waiting on a miracle | NASCAR News Now - SceneDaily.com
Based on this we can surmise:
They knew each other for years, been engaged for 6 months (as of the plane crash) planned to marry in December 2004 (no wedding plans in place just 2 months before the wedding? No engagement announcement, no dress purchased, no church booked...nothing...maybe they were eloping)
He references them meeting in Florida where both families own homes
He said she came to his house with her mother to tell them she was preganat and she was worried how thy would feel and they were worried how she would feel being alone (evidently they weren't close prior to that moment, and hadn't talked very much or they all would know how excited everyone would be)
H was suprised she wanted to stay in Charlotte and not be with her family. (I am too) He said it was because she had friends in Charlotte (she had only been there 5 months but whatever)
Then he says Emily and Ricky had been making plans to be together for a couple of years, so that puts them being "serious" when she was barely 16. He said they wanted to be together, that was their dream.
Take it for what you will because that is the only official story out there and eveyone envolved repeats the same thing exactly. No variations and no other details.