So you are trying to erase the contents of the phone, correct?
First, let's go to settings - iCloud and make sure "find my iPhone" is off. If not, turn it off.
Then go to settings - general - reset - erase all content and settings.
Now your aunt can set it up with her details.
Hope this helps. Let me know if you find any difficulty.
Did you mean after erasing all contents, it is asking to enter your Apple ID and password?
If you have removed your apple ID from Find my iphone, then there should not be any issues getting back in. Isn't it asking you to enter a new apple ID (or is it asking the password ***** an existing apple ID)?
I am afraid that phone was setup with a different ID and that is the reason it is asking you to enteer a password ***** an incorrect one.
Do you know whose ID that is?
sorry to hear that :(
If it is asking for an ID and password ***** set it up, I am afraid unfortunately only Apple can help.