[Of all things to divert the conversation he was not expecting this. It made sense, asking for proof of some sort. He was a complete stranger after all. Still, no outbursts? There's no reason to think that Jotaro would have a short temper about this but he had gone over the worst case scenario so often for the past few weeks that it's jarring in a way that it didn't occur at all. It was almost going too smoothly for something so out of the blue, it was almost surreal. Least of all being called family right off the bat.

Josuke lifts a hand to his shoulder, pulling the collar of his (normal this time, I swear) sweater down while turning so Jotaro could see clearly. There it is in all it's inexplicable glory, the Joestar birthmark. Once he's sure the other saw it he lets go of the fabric and shrugs his shoulder. He can handle questions like that.]

I've had one for as long as I can remember. [But he does understand enough of the Dio story he heard to guess what Jotaro actually means to ask.] I got really sick when I was four. That would have been the year you went to Egypt.