Faze, you are correct. This isn't for you to pay and your wife should work some additional time in order to cover this fine. Some countries have huge fines for not returning license plates, but they also send out notifications to the most recent address on that plate that they are due for return pending fine. The insurance lapse penalty is because if the plate isn't returned, the DMV will assume you're still driving with lapsed insurance until the plates are returned or reported lost.
It is INCREDIBLY irresponsible to not return these plates, and you should not pay for this failure to act on your wifes/families part. If I were you and required to pay, I'd get it in writing that you're doing it with the intent of being re-imbursed by the family (at $20 at a time if necessary) and that way if the marriage goes south, you can make a legal claim that they owe you a sum of money.
"All is fair in love and war." - Francis Edward Smedley