That’s why I have kind of put the blame on her. I expect it from FIL and BIL but not her I was really shocked.
Way to go OP, letting the males completely off the hook there. Just because someone always behaves arrogantly, selfishly and carelessly, it does not mean they get a free pass to behave arrogantly, selfishly and carelessly! Quite the opposite in fact.
If MIL is in a controlling, coercive relationship with a bully, she is very unlikely to have the emotional resources to stand up to that bully. If she did have, she would, but she clearly hasn't because she doesn't. I guarantee that, out of the three of them, she was the one least comfortable with the idea.
I agree with everyone else here that travelling in a car with your baby not in a car seat is completely irresponsible. But all the adults involved are responsible for it.
MN constantly chides men for their irresponsibility, yet here are many posters quite happy to ignore BIL and FIL and put all the blame on a woman who has been conditioned over years to give up and give in. The conversation you need to have is with all of them, not just her.
And don't be wet. I am one of the world's least confrontational, most anxious and people-pleasing people. But even I would find the guts to deal with this.