Drawing up an agreement only costs a couple of hundred pounds. Married couples tend to buy as joint tenants, which means if one dies the other gets their share of the property. Friends or siblings who buy together usually elect to be tenants in common. Especially when there's a piece of paper spelling out the terms of ownership. It costs an awful lot of money to get lawyers in after the event.
Prevention is better than cure.
That left the remaining person in real difficulties. You can have an agreement which says if one party stops paying they lose their rights in the property. Everyone says we should go to a solicitor to get a formal contract. But she's my sister. I think we'll play it by ear.