Because I think it will make him tough and resilient, I pick on my little brother all the time.
Your brother, what type of person is he?
He's a rather easy and normal person, that's how I perceive him.
Picking on somebody is not a good way to make them stronger. It hurts them and causes them to feel bad.
He'll be fine, an apology fixes everything.
An apology is a start, but picking on a sibling, or anyone, isn't a good way to toughen up a person. In fact, no one needs to be toughened up. You should love and support others, not be mean.