What you should have done many years ago, was to have a loan agreement signed by your family member saying there was a loan. Only then you can claim this payment as repayment of loan instead of a gift (no need to report). The loan agreement would have been very simple, just say who, when, how much, term of interest etc.
I would not suggest you to make one now and "back date" it to many years ago. That is forgery. You can write a loan agreement now and dated as today's date. To prove this loan was made, you'd better find the fund transfer evidence years ago when you mail the fund to your family member. And the number must MATCH EXACTLY as principle and interest listed in the loan agreement and the repayment, otherwise IRS might think it is a fake.
So for the reason stated above, it is easier you just file a IRS form claiming it is a gift. There is no gift tax anyway. Never be vague with IRS affairs.