Also want to add that if the brother has mental or physical condition that made him unable to support himself, he likely won't pass medical. And the whole application will be refused.It's based on your situation at the time of submitting the application not your status at the time when you submitted your interest to sponsor. That's what's determines who you can sponsor. The Interest to Sponsor is more of you saying "I want to sponsor to these people"...now you're submitting the application to actually sponsor them, so if you're situation has changed, you can submit the application for your parents only and maybe add a letter of explanation that your brother is no longer eligible. The door to him coming to Canada is not closed, he can always come here for schooling (and maybe move on to become a PR resident from there) but he will probably have a hard time convincing immigration that he's truly here for school if his entire immediate family lives in Canada.
That's something you'll need to discuss with your parents.
TLDR; Submit the application for your parents and determine an alternative way for your brother to immigrate to Canada.
EDIT: From the official source
Children 22 or older qualify as dependants if they
The only way you'd be able to add to the application is if you can prove number 2 and I could imagine you'd have to include some pretty strong evidence to support that.
- have depended on their parents for financial support since before they were 22 and
- can’t financially support themselves because of a mental or physical condition
The brother is young and you can financially support him to gain enough education and points to immigrate on his own.