I identify with a lot of what you have listed here as your symptoms.
I am also from a family with quite a few people suffering with a wide range in severity of ASD and AD(H)D and other unrelated conditions.
If you don’t feel like you can confide in a parent at this stage, is there another adult you could talk to? Aunt, Uncle, Teacher older cousins ect who might be able to help you navigate your feelings and concerns?
Alternatively you could ask to speak to your schools Special educational needs department.
If you didn’t want your school to know about your concerns about your self you could always say you just wanted more information to help you under stand your siblings better.
I am waiting for my formal diagnosis (prospered due to Covid) but I have not felt comfortable within my immediate family to discuss it until I have I guess proof because I know I wouldn’t have been taken seriously and if I had been belittled before approaching a doctor about it I probably wouldn’t have gotten as far as I have now in the process.