You may need an affidavit or letters from people that knew you were here then including your sister. However, at this point the officer has not asked for any other evidence. Your application may still be approved. If they issue a request for additional evidence you will need the letters. USCIS can also refer your case to an immigration judge. The judges can be much more reasonable. DHS and the former INS have a computer data base that records entries and exits - you obviously had an entry and no recorded departure. Relax and see what their decision is.
The answer provided here is general in nature and does not take into account other factors that may need to be reviewed for a more precise answer. You should consult with an immigration attorney before taking any action. The answer here is not intended to create an attorney-client relationship.
You have invested 13 years in this case ... why not invest some $$$ and pay an attorney to help you develop affidavits, etc.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- firstname.lastname@example.org -- www.capriotti.com -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
Your I-94 should have been sufficient since you had just entered and they have no record that you had left the country by December 21. Anyway, the officer will not deny your application without giving you an opportunity to provide more evidence. As for the I-94, what the officer did was normal.
You have a lawyer who knows the facts of your case. You should listen to his or her advice. if you have lost confidence in your lawyer schedule a consultation with another experienced immigration attorney for a second opinion. Your lawyer should have known to address these issues from the get go.
While this answer is provided by a Florida Bar Certified Expert in Immigration and Nationality Law, it is for general information purposes only and an attorney/client relationship is neither intended nor created. You should seek out qualified counsel to review your case and provide you with advice specific to your situation. Call +1-561-478-5353 to schedule a consultation with Mr. Devore.