Not only is she a dick for putting her OWN CHILD at risk of serious injury or death.
She put YOU at risk of getting done for HER child not being LEGALLY restrained.
I've had similar issues with my sister in the past (now Nc this was one of the more minor issues!) I reached a point I refused to drive her or her DC unless everyone was properly restrained. The DC actually didn't have a problem with this at all as they're used to their dad insisting. Sister is an arrogant twat who thinks her own driving so great she'll never be in a serious accident.
In my case this lead to 2 occasions where planned days out were affected. My dd didn't miss out, hers did which is unfortunate, but I was clear with both sister and mother (she's very much mum's gc) that it was HER behaviour that lead to this not mine!
In your position I would absolutely be telling her VERY clearly that if she wants you to drive her & your niece anywhere then the rules MUST be abided! No compromises.
YOU'RE the one taking the legal, moral and practical risk of her child being unrestrained in YOUR car.
I'm also willing to bet if there WERE an accident and your niece was unfortunately hurt - even only a minor injury - YOU would be blamed forevermore!
On the parenting side of things she absolutely is creating trouble for herself. Part of parenting is setting boundaries, sometimes making kids do things they don't want to FOR THEIR OWN GOOD. If she can't manage a 2 yo she's screwed come teens!!!
I've been in a major accident myself I'm so thankful dd wasn't in the car. My car was stationery when I was hit from behind by a guy pratting about on his phone! Who didn't realise the traffic lights were on red. Shunted me into oncoming traffic meaning another driver (not their fault) hit me side on - the side dd would have been on! In the scenario you describe if that accident had happened with your niece in your car chances are at the very least she'd have lost an arm if not worse. Neither were fast roads either I was on a 30mph road and phone twat at least wasn't speeding! Road I was shunted into was a 40mph road. So again not a fast road - and you I think were on a motorway? So at least twice the speed involved? My car was a write off, the others sustained significant damage, I suffered whiplash, bruising and later discovered a spinal injury (complicated not immediately obvious on X-rays etc). Which has left me permanently disabled with pain & difficulty with mobility.
I'd be sending her videos of crash test studies as previously suggested particularly ones relating to the conditions you were driving in - because I believe if you had continued and there had been an accident at those speeds your niece would have been at high risk of death!
Joking aside I personally think that now we have the tech where this would be possible we SHOULD have cars that WON'T go until everyone's strapped in and the drivers passed a breathalyser!
Some sobering reading here:
https://www.theaa.com/publicaffairs/reports/looseeintheecar.pdf
There's even now starting to be dash cam footage of what happens in accidents. Not dummies but real people including children really being injured/killed.
LettuceP actually if the child is ABLE to get their arms out the straps the straps aren't correctly fitted. It should be impossible to do so. There's also gadgets available to add to too easy for child to undo clips although I think clips are being made harder to undo now too.
Excellent reply op. My sis is also one who CANNOT take the slightest criticism. So I'm not surprised you say yours is the same - that's why she can't cope with saying no to her child too, but it does make her a crap mother frankly!
All you can do is look out for yourself and not give in to her nonsense.