I would certainly sit down with my aunt to discuss the situation. In order to have this gift, you will most definitely need proof of funds from your aunt's bank. No one can do this but the account holder (your aunt). This is why I believe sitting down and having a heart to heart with her is a good idea. Then I suggest you get preapproved by a bank and let them draft a letter stating that you will need this documentation in order to show your source of downpayment. With all of this spelled out and in writing, if your aunt is still hesitant to "show you the money" then I would wonder if she truly does intend to follow through.
While I know that it's hard sometimes to reach family members when it comes to money issues, this is an instance where you really do need to know and have documentation of where you stand.
Hope this was helpful to you.