The danger in not having a written agreement documented when it comes to issue with money is always something disastrous and regretful in the end. I read similar story minutes ago and the mistake was that there was no written agreement even when he was told to take one but he refused because he allowed emotions in it. This is something one should be careful of in business. There is no family in business at all.

Meanwhile, I like your sister. She understands what business is and how it should be run.