Thank you and congratulation to your parents. As i have seen questions in relation to dependent child popping up here and there in the thread, I think it would be good if i sumarize your case here. It would be benefit for other ppl in the same situation like you.
1. Age restriction:
Any application prior to 11/12/2016 (date when the law changed, not quite sure if it is correct) will not have any age restriction.
Your bro was 22 at the date of application and 26 at the date visa granted.
2. Dependency check:
The co will ask for further evidences of dependency at the time of processing.
Your bro was still a full time student at the time of processing, hence still be dependent on your parents. Could you please share your experiences of how to prove the dependency. Which docs have you provided?
3. Aos only provided for your parents
You only need to provide ao for your parents not your bro although he is an adult too. Am I correct?
Feel free to correct me if i am wrong.