Share your insights or perspective on the financial matter presented in the input.

How do I calculate ownership percentage for shared home ownership?
The bottom line is that you can decide whatever you want to do.  It is good of you to get everything in writing.  What happens if she decides to move to a different city?  What happens if she also wants to be bought out?  It should also include contingencies for your husband and yourself.  God forbid anything negative happens, but what happens if you two get divorced?  Does your husband want to be an agreement with your sister if you pass away? There does not seem to be any math to do in this case.  While she is paying the lion's share of the payment, she is also receiving the benefit of having a place to live.  It is unlikely that she can rent an equivalent place for anything close to 1400/month.  I would estimate it would be at least 1800/month to rent an equivalent property. So she put no money down, and she is paying below market "rent" to live somewhere.  Many people would be happy to have $400/month off and handle their own repairs (let alone you still kicking in half).   Now all that said, if you want to give her some equity based upon generosity or the desire to give her some dignity, then you are free to do so.  Perhaps 10%?