Brief History

During the beginning days of the Fantastic Four, Ben Grimm, aka the Thing, used to preach about "his Aunt Petunia" very often. Whether there was danger or Ben was just relaxing, he often called himself "Aunt Petunia's favorite nephew". None of the team-members really payed any attention to it, not more then Ben's other battle cries "Its globbering time" and the likes.
That's why the rest of the team was extremely surprised when a younger woman walked into the Baxter building asking for Ben. 

When Ben finally saw her, he ran up to her, saying, "Aunt Petunia" and closed her in his arms. So, it turned out that Ben really had an aunt named Petunia.
Petunia lived in a small town, and first came to Ben after he had changed in order to ask his help to stop alien creatures from running havoc in her hometown. A young girl named Wendy who lived in the same village had befriended these creatures, although nobody knew this. The FF couldn't explain where the creatures came from, but they did help stop them from terrorizing the village. Since then Petunia has been a trusted sight at the Fantastic Four headquarters. 
Some years later the FF fought the so called Masters of Doom, whom apparently killed Petunia. Although nothing is known about a funeral for Petunia where the FF went to, so it could have been just a mirage the Masters of Doom used to frighten Ben.
 
 
 
Trivia
Although Petunia is already mentioned as early as in Fantastic Four vol.1 no 26, and numerous times after that, it would take more then 200 issues to finally see her in person.