As always there is an exception ...mio, tuo, suo, nostro, and vostro.. do not have the definite article in the singular. Loro is the exception and takes the article, so il loro fratello is correct http://italian.about.com/od/grammar/a/italian-possessive-adjectives.htm (about halfway down)
I think it shouldn't be mandatory to use the article. Anyway, with "il" it sounds better, I cannot explain you why. If you have more examples you don't understand, please list them and ask me again, we will try to find reasons behind what is more common in Italian.
Report it, anyway, without "il" is as correct as with it.