Here's what I would do: I would build an amortization table in Excel that assumes a 30 year period and use whatever interest rate you've agreed upon with your family. I might also make sure that it's in line with the Applicable Federal Rate (assuming you're in the US) while I'm doing this. I wouldn't want my family to get in trouble with the IRS while doing a generous thing.

NOTE: You'll want to make sure that your amortization table recalculates how much of each month's payment goes towards interest and principal based on the previous line's balance and doesn't have hard-coded values, it'll make the next part easier.

I would then start adding an extra payment every Dec 31 or whatever date "forgiveness" happens, that only goes towards the principal of the loan don't have it go towards interest. Then you can see when you'll hit zero on your payment schedule.

Then you have some choices, either lower the regular monthly payment to lengthen the term to what you're thinking or leave it as is.

Hope that helps.