Are Steroids Addictive?

Yes, steroids can be addictive, like all things that give pleasure or help to avoid pain. When that occurs, parts of the brain are stimulated to generate new programs, so to speak. Drive and desire are created, sometimes so powerful that they can take control. These addictions can overpower a person's conscious will or seemingly enslave the person to satisfy those drives and desires, even when it is against the person's best interests. Addictions can be chemical, as they are with painkillers and cocaine, or they can be behavioral, like gambling or Internet addiction, though all addictions are really chemical in nature when you take into account that the operation of the brain is conducted by neurochemicals.  

If your brother is intelligent enough to understand the risks and damage he is doing to his body with steroid abuse, but he continues to do it anyway, assume that the brother you love and respect is no longer in control. The addiction is. When an intelligent sane person realizes that what they are doing is hurting them, they stop it. When the attachment or addiction is strong enough to overcome their intelligence and will, that's the "insanity" of addiction.  

It sounds like the rewards of using steroids have your brother hooked, addicted. You may want to respect your brother's decisions over his body, but they have been rendered irrational and unhealthy. Your love for your brother would be better expressed by helping him to face the fact that he is addicted to something that can do irreparable harm. Get other family members involved and see if you can get him into treatment.