You may be able to claim the expense as a medical expense, but this is a complicated topic and you may need to consult directly with the CRA.
The first thing you need to determine is if your brother can be considered a dependent of yours. The links below may help you decide that:
https://turbotax.intuit.ca/tips/who-are-considered-dependants-5145
https://www.canada.ca/en/revenue-agency/services/tax/technical-information/income-tax/income-tax-fol...
The second thing you need to determine is if his rehabilitation program is considered an "eligible" medical expense. Depending on the nature of the program, eligibility might differ between the provinces. It also helps if you have a prescription or some kind of "official" document saying the program was medically necessary.
Here's some more information from the CRA.
https://www.canada.ca/en/revenue-agency/services/tax/technical-information/income-tax/income-tax-fol...
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re...
