How much do I pay my lender in interest

[Rich Hupper]

Hello

I am buying a $300,000.00 home with 55k down. 

I plan on selling in 2 years. My sister has agreed to give me money for some rehab needed in the home. 

She has agreed to let me borrow $50,000.00 for 2 years @15% interest. 

Do we have to create a mortgage where she would be a second position mortgagee?

How much interest would I owe her if I borrowed that money from her for exactly 2 years?

How would I calculate this?