If you don't mind me asking... why is it the brother does not (or did not at the time of the post at least) know that the test was positive? Are you saying he don't know he did the test in the first place? How on earth did they make a blood/saliva/urine test that he wasn't aware about? Did they lie and say that the test came back negative? O.o
It's a hard question but i agree with Hurricaneclaw. talk to his parents. Descide with deep conversation who should tell him; his family and/or you. You're right in thinking he shouldnt be in the dark on this but, with that said, find out if you should tell him or his family. Some people may feel it a touchy subject for even their BEST of friends to know...
I think is not your duty. Even of you are a good friend, this kind of things are a family thread. I suggest you to talk with your friend's brother and his parents about the moral struggle you feel, you'll find a solution. Be strong, i wish the best for your friend's brother.
I don't believe it's your place to tell him. If your family is close with his family, then perhaps you could talk to his parents about it, or ask your dad to talk to them. It really seems like it's up to them to let him know. You can be there to support him when he finds out.
It is best that you let his parents tell him. You can tell him, but then he might get mad at his parents for not telling him, and right now they need peace among themselves so as to give his brother all the support and love that he needs. What you can do, is talk to his parents about telling him as soon as possible - it isn't right that he doesn't know yet. Other than that, you can only pray for them, and support your friend as much as you can.
Honestly? Not your family, not your business :/ I wouldn't like my friends telling my family anything without my permission, personally. You have no moral obligation to tell anyone anything, unless it was you knew he was living a "loose" lifestyle and other people were at risk. Then you'd call the cops. But if he's sick and hasn't disclosed his illness to his family and hasn't asked you to, then it's not your place to say anything. Hard as it is.
I'd stay out of it, he's gonna know with or without your influence and in the case of an ill family member who is likely to get worse before he gets better I wouldn't want to chance making him feel like his family betrayed him somehow by telling you and not him. if you let it be and let his family handle it he will feel like they waited to tell him until he/they were ready and not feel the kind of resentment towards his family he might feel if you are the one to tell him.
Is there something preventing his brother from telling him? (Like being in a coma). If that isn't the case, I don't think it's up to you to tell your friend. That's up to his brother, and maybe his parents, but not you. You could talk to your friends' brother or his parents why they haven't told him yet, and ask them to, but it's not your place to reveal a secret they've kept from him for whatever reason.
I don't really know your life situation, but as a longtime friend of someone who has HIV I don't think you should be the one who reveals the illness. It should be up to them to say so when the time is right. Unfortunately in this miserable world that condition carries an extra stigma. And the fallout family wise can be unpredictable. Which is why I urge you to hold out telling for now.