As someone who has the condition myself, I'm happy to add to the discussion thus far.
I haven't applied for a Class 1 yet. I do however, fly gliders and my GP signed me off, the gliding club (who know of the diagnosis) were happy to send me solo. From what I'm aware, it is possible to obtain a Class 1 even after a confirmed diagnosis, depending on the individual. Therefore, the OP's twin having it shouldn't be a problem.
I think the OP would be best off declaring it at the medical, I don't think one can be too honest really. Autism most definitely isn't a mental illness.