Yeah, it was just the youngest boy that was the hybrid. It sure would have been nice if they would have showed which tests they put the boys through to see what he passed to convince Sam & Dean that they were human.
Some of it might have been that they were orphans and Eve even said she did that on purpose to try and have them gain their sympathy, so maybe they skimped on the tests.
I'm on the fence as to whether Eve is gone or not. I sure wish they would quit killing off all the hot demons/monsters