Borrowing money from family member to purchase existing business. Best approach

I have the opportunity to purchase an existing profitable business which is also a franchise. The only way I can purchase this business is to borrow money or ask my sister about investing. She would be putting in about 90% financially, but I would do all the work. The franchise fee is 9% and this business is profitable with growth potential. The existing owner lost the time she needed to run it and is busy with other projects. What should I do? If I just get a loan from my sister how can I be sure it's a good deal for both of us? Should I just have her invest? What's a good plan? The amount I need to borrow is 48,000 but the business has a potential of 65,000 annually.