Fair way to set up joint PI

I would like some advice on a fair way to set up the following investment.

My brother purchased a property off the plan 2 years ago (Mascot, Sydney) paying 10% up front which he intended to live in, his circumstances have changed (married child etc) and now the unit no long suits his lifestyle but is due for completion in March 2014 in which case he now needs to get a mortgage for the remaining 90% owing. He would like to keep the property in Sydney as an IP but he is concerned that even once rented he may along with his new home be unable to pay both mortgages and maintain his desired lifestyle.

My question is he has asked me if I would consider going in with him as a joint IP but I wondering how to set it up, could for instance my brother keep his 10% share and we split the remaining 90% or is this unfair for either me or him? I have a lot of extra income but no deposit and he has said this may suit him. I don't want anyone to feel short changed that is why I am asking. I do like the property, price he paid, location, returns etc.