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How do I calculate ownership percentage for shared home ownership?
Sister is putting down nothing, and paying sub-market rent. It looks to me like if she is assigned anything, it's a gift.  You on the other hand, have put down the full downpayment, and instead of breaking even via fair rent, are feeding the property to the tune of $645/mo.  In the old days, the days of Robert Allen's "no money down" it was common to see shared equity deals where the investor would put up the down payment, get 1/2 the equity build up, and never pay another dime. This deal reminds me of that, only you are getting the short end of the stick.  "you never think something will cause discourse" - I hope you meant this sarcastically. The deal you describe? No good can come of it.