P.S. you could always take the bull by the horns and confront your family, this could be festering away inside them too. And who knows medically in the future how a sibling could help out or vice versa.
I agree with amberdog. Life is too short for all these pretend delicacies.
If it were me, I would write a letter to every family member who was old enough to have any information. Tell them that you have put up with this nonsense for far too long and now is the time to end it. Remind them, in no uncertain terms who's brother you are talking about and demand to know (from each of them) what they know or possibly know.
SMS messages, texts, emails and the like are far too easily ignored and discounted. A letter arriving through their letterbox will have the same weight and power as a final demand from the tax man - almost impossible to ignore. Until you get the information you want, stop calling him your half brother. Call him your brother. Half brother gives them wiggle room. "Oh, he's not really one of us", type of thing.
You will have to be bloody-minded about this - make it your only topic of conversation till you know the truth. "Yes, it is a nice day, auntie, but what about my brother?" If you carry on long enough, one of them will break down eventually, with something like "For goodness sake, he drives the No 37 bus for Jones's Coaches over in Nexttown."
Remember that they are the guilty ones; they are the ones suppressing the truth - make them feel guilty - they will feel so much better once the burden is removed.
Regards
Chas