A loan from a relative

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Hello everyone. I'm currently working on buy my first rental property. I have a relative with some disposable income. She is looking to invest, and said she'd be interested in funding some of my deals. I would need about $25,000 to 30,000 from her to close the deal. If I were to promise a return on her investment, how would I determine what's fair? Also what documents should I use to promise her that return on investment?