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"have to" is a very vague term. One does not "have to" follow the law at all.
In this case, one either has to charge interest, or pay interest. Either way, from what is written, the IRS requires interest to be charged in some fashion.
3 months, 2 weeks ago on Family Loans: Does The IRS Care If I Lend My Kids Money?
Does one have to charge interest if it violates one's religion?