Definetly a sin, probably a mortal one.
Oh, and a crime, too.
You are being paid to commit insurance fraud. Your part in the scheme is effectively to lie to the company on behalf of a family member.
This isn’t like letting him/her copy from your Algebra exam. The info in the driver’s course could potentially save a life, his or someone else’s: that’s the moral responsibilty you are taking on to your own shoulders here. What is this family member doing that’s more important than that?
And its VERY pre-meditated. He agreed to the course to save a couple bucks, figured it would be cheaper to pay you to lie about it than to accept the consequences of his actions. That says something pretty ugly about him, and about who he thinks you are.
Note I say ‘probably’ mortal sin, not definetly-- consult your priest, etc. But you have clear intent here, no emotions, hormones, etc. to dull that edge. You are fully consenting, there is no gun to your head or other mitigating factor, and the matter is serious: insurance fraud. The amount of money may be small, but that just makes you a cheap ‘date’. A five dollar hooker is still a prostitute.
Reading back over this, it sounds far more harsh than I mean it to, and I’m very sorry for that: I wish I had the skill to convey my points in a more gentle manner, but the points remain valid.
Maybe I’m overstating the case: it’s less serious than helping the family member lie about an injury, or providing him with a fake alibi for a serious crime. But it is very much the same as providing false documents so they can collect money they don’t deserve: it’s exactly like that.
I’d simply tell him that you are awfully sorry, but after thinking about it, you just aren’t comfortable with the situation, and need to spend your time more productively.