Well, I hate to say it, but you're not going to.
The problem is that they're not just buying 50% of an investment, they're becoming equal partners with your brother, who is a stranger. Plus, they'd have to come in with cash or an unsecured note - your brother probably doesn't want a stranger to mortgage the property as a means of buying you out.
You are better off working something out with the brother - hiring a property manager, selling him your share on a note, something, anything other than wait for the needle to show up in the haystack with a light shining on it and a little bell signaling its presence.
All the best,