My brother is a foreign student on F1 visa, he is 20, lives with me since Aug 2015. I am his sponsor (college, food, health, everything). Can I claim him as a dependent?

[christiandika]

I got a legal document made in Cameroon (our home country) saying parental authority on my brother was given to me.

[LinaJ2018]

If he is not a resident of Canada or Mexico, you cannot claim him.

As a F-1 visa holder, he is considered as a nonresident and 'exempt' for not counting days when he resided in US for five years.  In order to meet the dependency rules, the qualifying person must be a U.S. resident alien, U.S. national, or a resident of Canada or Mexico.

https://turbotax.intuit.com/tax-tools/tax-tips/Taxes-101/Claiming-a-Non-Citizen-Spouse-and-Children-...