So this is a loan?
This sounds like an opportunity that could be good. But what happens if something goes wrong? I'd be very wary of taking family loans. Depending on the family, it can range from uncomfortable to disastrous if you can't pay it back. That's a very nice offer but the fact is you don't need it to succeed. You're young, have a great start, no debt, and if you go mustache style, you could be financially free in 10 years either way. I think your hesitation is wise--wiser than I was at your age.
I'd suggest if you do it, then be prepared to pay it back on time, even if your deal doesn't work. So if you buy a house to remodel and its value crashes, be ready to make up the difference out of your income. That may mean living in a shithole apartment and eating rice 7 days a week.
So, maybe you mitigate the risk by borrowing only a small amount.
Or, maybe you could live now in a shithole apartment, eat rice 7 days a week, and increase your savings rate to 55% to save enough of your own money to fund your dream in a few years.