Who's Purchased a House with a Sibling?

[Cam Hadfield]

Considering a property with my brother.  We're like minded in terms of work ethic, finances, and investment strategies.  I currently own a duplex on my own, I live in one side and rent the other.  We'd both be bringing a stable income/job to the table, he'd bring most of the down payment, and I have the homeowner "experience" and am more than dangerous with a good toolbelt.  I forced about $40k of equity (~10% of current valuation) into my duplex in the first year, lots of fun projects.

At the moment I'm not concerned with ruining the sibling relationship (naïve perhaps, I know), and want to keep this conversation about the business side of things.  We'd actually be living together in this house, and I'd exit the deal in the next few years - pull cash out for a second (third) property, he'd likely keep the house. Or perhaps I'd stay and he'd cash out.  We've worked together in a professional setting in the past, are very close, and he's the first person I'd call for anything major in my life.  To the extent that this work would constitute a business relationship, we both want it in writing and enforceable.

Tell me all your horror stories.  What could go wrong in any business relationship?  Extra concerns over the family relationship?  Is it difficult to define the distribution of responsibilities or equity?