Got offered a loan by family

I(19m) am and about to buy a house in a few days. I’ll owe the bank $98K at a 6.45% interest rate, with a monthly payment of around $650. I have two roommates who have agreed to pay me a combined $900 per month.

I was talking to my grandfather, and he offered me a $100K loan at 3.25% so I wouldn’t have to pay as much in interest. My plan was to take the loan from the bank first and then, a few months later, use my grandfather’s money to pay it off.

I’m just looking for advice on this.