About that relative he might have, that would actually be a descendant from Twilight Princess and it is sort of an easter egg tying back to OoT. So sadly, we can't really use this person to get any closer to the truth as she also doesn't know why she gets itchy sometimes.
Noise alert. The audio static is strong with this recording I found on YT.
It definitely is interesting to theorize how come he has such a strong itch.
For starters there's something called "swimmer itch" caused by bites of certain parasites from contaminated waters. Even though humans are not the usual hosts they can get in the skin, and I don't know if this is related or not but one of the shapes are the flat worms. It is probably just a coincidence but the sinking lure kind of looks like a worm, so, who knows, maybe that lure belonged to the fisherman and he actually has been spending too much time in areas without properly washing and drying himself afterwards. So yeah, maaaaaybe Link shouldn't be using the Iron Boots when he goes fishing on that pond in Lake Hylia.
I WORRY ABOUT YOU, FAIRY BOY!
I already feel itchy just seeing this water level...
That being said, it would be odd Hena also shares the same itch problem. If this was to tie her as a descendant, then this suggest the itch could be more some sort of genetic medical condition. Perhaps this condition gets exacerbated in this line of work these two share. Hahah.