This isn't going to get anywhere as a complaint.

I totally get why you'd try to travel without any special assistance and just be one more family group on the plane without anyone needing to be aware of the special circumstances around your brother.

But, this is one possible consequence of doing that, and you don't get to play the 'Disabled' card when you've done everything possible to disguise and hide that fact from the FA and the airline.

Your description of the events reads somewhat hyperbolic and the FA motivation could be anything including simply making sure the passenger who had not interacted with her at all, had not been 'allowed' to eat his meal etc. was in fact OK...

Allowing the airline and their staff to be aware of any special needs or circumstances lets them respond and act more appropriately to what otherwise may raise concerns.

If you deliberately deny them that knowledge you need to be way more understanding when people jump to the wrong conclusion and then act in a way you find problematic purely because they lack the knowledge you withheld.