Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

How to divide a mortgage and living area fairly?
I think what you have here is actually TWO agreements with your sister, and explicitly splitting it into two agreements will bring some clarity. The first is ownership of and responsibility for the building. The second is each of your personal use of a unit.  Here's what you do: Treat ownership as if you're not living there. Split the down payment, the monthly mortgage, taxes and insurance, responsibility for cost of maintenance, etc. as well as the ownership and benefit of the building 70%/30%. Put all that in a contract. Treat it like a business. Second, lease those units to yourselves as if you were tenants. And yes, I means even with leases. This clarifies your responsibilities in a tenant capacity. More to the point, each of you pays rent at the going rate for the unit you occupy.   If rent from all three units equals the monthly expenses, nothing more needs to be done. If they're more than the monthly expenses, then each of you receives that as business income on that 70%/30% breakdown. If those three rents are less than the monthly expenses, then each of you are required to make up the difference, again at 70%/30%.  Note: if any of those expenses are utilities, then they should be apportioned via the rent -- just as you would if you'd rented out the whole building to strangers.  2nd note: all that can be done with ledger entries, rather than moving money around, first as rent, then as expense payments, then as payouts. But, I think it will benefit all of you to explicitly pay rent at first, to really clarify your dual relationship as joint owners and as tenants.  Final note: I think this is a stickier situation than you may think it is. Familial relationships have been destroyed both by going into business together, and by renting to family members. You're doing both, and mixing the two to boot. I'm not saying it will destroy your relationship, but that there's a solid risk there. Relationship destruction comes from assumptions and vague verbal agreements. Therefor, for the sake of all of you, put everything in writing. A clear contract for the business side, and clear leases for the tenant side. It's not about trust -- it's about understood communication and positive agreement on all important points.