You (should) remember that while applying for PAN they ask your anme NEXT how you would like your name to be printed on the card.
Your aunt's card might be bearing the name she likes to have on the card. This "may not be" acceptable to site while registering. So you should know her full name and how did she fill the form i.e. first name, middle name Last name etc.
If you can't / do not want to contact her for this simple thing here is a short cut method.
From one site or the other go to payt ax online there fill the PAN No. on the next screen your aunt's full name appears. Depending on this you can decide what could be the first name, middle name, Last name
then fill up.
all the best..