If you are buying a house with someone who does not share their financial life with you, you will need to consider several extra points in addition to where and how much. The first question is how to take title [reference last post]; the second is how you run the day-to-day and the big questions of when to sell or refinance.
Whenever I have clients going on title to a property with a sibling or parent, I recommend they get a trust agreement that details their obligations. It discusses who is responsible for which expenses (mortgage, taxes, insurance, maintenance, etc.), who can authorize a new mortgage, and when the property can be sold (and how much each party gets on a sale).
Having the rules set out clearly at the beginning makes for far less stress going forward.