This theory seems to hinge on Ash's ineptitude as a trainer compared to Red.
Now I don't agree with this premise. We need to understand that Ash and Red occupy virtually different universes- the anime vs. the game. The game has very easy rules, while the anime is much more realistic and obviously harder. This is why Ash seems inept to us, while in reality he's just operating in a different, more realistic world.
This aside, other premises that the pasta uses to argue for the brother theory:
- Ash's Pikachu is very overpowered (which is explained by saying it's Red's)
- Brock and Misty quit their gyms (in order to look after Ash)
Seriously, one would require an extreme stretch of imagination to get from these premises to Red being Ash's brother. Conspiracy theories about pokemon are interesting, but only when their premises are interesting and surprising. This...just seems too stretchy for my taste.
"Perseverence in science needs to be tempered by flexibility."
-Haig Kazazian, in Mobile DNA: Finding Treasures in Junk