I couldn't see why not..

if your parents agree to give him up for your adoption..I also do not think that citizenship will make a difference. Even if you are a chinese citizen with no greencard, you can still adopt under chinese law. Once legally, your brother is your "son," he could still come under somekind of visa...

I am not practicing family law...however, if you believe that you will be able to provide a better life for your brother, you shouldn't stop here...