I'm not an AME, but i'm a medical practitioner. I wouldn't be personally concerned if some of your family members had autism - doesn't mean you would be affected.
If they ask you to declare if a family member has a mental illness, I probably wouldn't declare autism or aspergers. Why? Well, in the strictest sense, mental illness usually means psychoses (e.g. schizophrenia, bipolar etc) or neuroses (depression, anxiety, OCD etc) and autism doesn't belong to either class, though very often people who are autistic might have concomitant neuroses, e.g. a lot of autistic people also have obsessive compulsive disorder.