To answer the question at he end of the post, yes YTA for intentionally making your brothers meltdown worse. That is not to say his actions were right, however I will explain.

First of all, OCD. OCD is not just being obsessive just because, most of the time the person is entirely and utterly convinced that if the compulsions are not met something horrible will happen. Someone might be convinced that if they don’t flip their light switch on and off three times before bed they’ll die in their sleep. If they don’t lock and unlock the door five times robbers will break into their home. Your brother wasn’t just asking you to sneeze because he liked the neat number of three, he was asking you to sneeze because to him if you did not something absolutely awful would happen. Would you not try and force someone else to sneeze if you were convinced if they did not sneeze again your entire family would die? Or something similarly awful. Obviously this is not a rational way of thinking and it is incredibly disruptive, hence why it is a disorder. But acknowledging it as such does not make it not exist.

Now onto his meltdowns. He has these meltdowns for a reason. Something that he has no control over is upsetting him to a degree that he goes into that state. He has no control over this. He cannot stop being autistic to accommodate your feelings. You, however, can do things to help him out. But instead of doing this you opt to actively make his meltdowns worse as revenge for the inconvenience his disability puts on you.

So, to sum, are some of the behaviours caused by his disability really annoying, inconvenient or harmful? Yes. Can he help that? Not as much as you’re expecting him to. Are you undeniably the asshole for deliberately, while having full control of your actions and not being influenced by a condition like your brother, forcing your brother into an extremely distressed state and actively trying to make it worse because his disability inconveniences you? Yes. Absolutely yes.