LOL In my haste to correct you MrsL I misread--so I did state that wrong before.
When you read the charts, it states the laws PERTAIN TO CHILDREN UNDER 18. 16 states have primary enforcement for UNDER 18.
So not your MIL.
You're still welcome to use your legality argument but its still WRONG.
I think you need to go back to elementary school math and learn the "greater" than and "less than" symbols. You seem to need to be right so go right ahead with whatever you believe, but according to the chart, the text above, AND the government websites of the individual states, many areas have laws that include adult passengers.
Aside from the legal issue, in my car everyone wears a seatbelt. Period.
You brought in the legal issues because it was going to be justification. You can't give the information and then just dismiss others' concerns with it. You do come off as VERY preachy and self-righteous. Also, you can't say "everyone wears a seatbelt in my car" and then say "oh but it doesn't make sense for her to take her own car if she doesn't because it's a long trip," NO. Stand your ground. If she doesn't want to, she doesn't go. Don't go justifying it. Just be forward. There was nothing wrong with any of the advice given to say put it on or get out. There doesn't have to be a "better" way to say it.
I agree with PP that you need to have a conversation... In a perfect world, your DH would bring it up and deal with it.
DH never wore a seat belt before we got together. I'm anal about seat belts. I told him (when I was pregnant and he unclipped his belt 3 blocks from our house) if he widowed me, I'd name our baby Betsy. (A name I LOVE but he can't stand.) He was like "everyone would know I hate that name." And I said, "And everyone know you don't have a say anymore because you couldn't keep your seatbelt on."
Re: MIL troubles
You do come off as VERY preachy and self-righteous.
Also, you can't say "everyone wears a seatbelt in my car" and then say "oh but it doesn't make sense for her to take her own car if she doesn't because it's a long trip," NO. Stand your ground. If she doesn't want to, she doesn't go. Don't go justifying it. Just be forward. There was nothing wrong with any of the advice given to say put it on or get out. There doesn't have to be a "better" way to say it.