Well..

She hurt her back pretty bad. You plan to take her to the hospital, but it seemed her spine was damaged severely. You remember the spell Tonks used on you when you broke your nose. You pull out her wand and pointed it on her back saying, "Episkey". In that instant, you heard a loud crunch and a pop, which made Petunia scream loud. Then she was healed. However, you wish to take it a bit further.

"Aunt Petunia I have just mended the back temporarily, it needs to be taken care of diligently. Why don't we retire back to your bedroom?"

Your aunt nods and you start carrying her (Again thanks to Quidditch training) and swiftly moved her to her bedroom. Your aunt then croaks, " This is the second time you saved my life, Harry. I know I have treated you harshly before and I apologise for that."

"No worries, Aunt Petunia. I guess I should retire."

"Harry, wait. It seems my back still sore from the fall, can you take the lotion and spread it on my back?"

What a wonderful opportunity, this is the only way you can subdue your aunt. You agree to the task and Aunt Petunia rolls over while pulling down her robes till her buttocks( a pity, no matter anyways). You pick up the lotion and you spread it on her back, then you take out your cock and on your left, you shag yourself and on the other, you gently rub her back. You shot quickly due to the buildup in the previous event.

Now you mix your cum and the lotion on her back, which made her aroused even more. She moaned loudly forgetting the fact that you were behind her. You continue to rub as you slowly pull off her dress to reveal that her panties are soaking wet. Since the effect of your cum is still on she didn't care that you were rubbing her pussy. You cock began to rise again and using the lotion you lube yourself to get a larger effect on her. You start teasing her pussy with her cock which caused her to orgasm again and again.

Petunia then removed her panties and spread her legs still showing her back to you, giving you the opportunity to make her your **** once and for all.