Sheldon having a dissociative disorder would explain certain inconsistencies. E.g., why he was allergic to cats in just the third episode of the series but obtains a good sized collection of them in The Zazzy Substitution without any problems, just 3 seasons later. One alter is allergic and one isn’t. (Being able to pull this off is something most people can’t easily do.)
(This is starting to sound familiar. I think I’ve proposed this before.)
I used to have an extreme allergic reaction to shellfish, but now i can eat the stuff without incident, so it is possible to outgrow an allergy.
I don’t know how well-adjusted his siblings actually are, but I wonder if his mother treated him as some sort of social experiment. In one episode,he talks about the differing reactions his mother had to his science experiment about lima beans, and he says “If you’d like to look at the relationship between nurturing and growth, I’d like to point out that my brother is eight inches taller than me!”
ETA Should have read further - I see it’s already been suggested.