My sister-in-law wore a helmet

So my wife has a brother(D) and sister(AM), and they were here for a visit not to long ago. When they all got to talking it came up that AM pretty much doesn't remember any of her childhood. Because of this fact D thought it would be cool to convince her that she wore a helmet when she was kid. So the running joke is now that AM wore a helmet when she was a kid.

Yesterday afternoon my wife and I were watching TV when one of those stupid "Incredible Edible Egg" commercials came on. My wife then tells me a story about AM when she was about 5 or so and having seen such a commercial goes into the kitchen and gets an egg. She then walks over to my wife and says "The Incredible Edible Egg" and then smashes the egg on her head. To which we laughed about it and I said "Did she crack it on the helmet she wore?". To which my wife almost lost bladder control and then called her sister. Her sister called me an ass, but insisted that my wife tell her soon to be husband that story. So my wife tells the story of AM and the egg being cracked on her head. His response, without telling him my response first, was "Is that why she had to wear the helmet?" To which my wife again almost lost bladder control.

Now if only I could convince D to buy AM a helmet for her wedding present....