Except, it is of course not disrespectful in the slightest to simply sit in your own seat in your own car.
You're not the only one with this strange belief, but you are certainly in the minority.
So, let's examine the situation.
Even if you actually believe this strange notion that booting someone out of their chosen seat is somehow normal or expected, or that the OP should have simply chosen not to sit where she was comfortable and happy to accomodate the woman they were doing a favour for - what happens if they don't move?
Do you scream abuse and call your son's wife a bitch, as the mil did? Or do you just shut up, accept the favour, and get on with your life?
Are you going to pretend that it's not obvious, given the mil's reaction, that she is an overbearing person used to demanding her own way and for that reason alone should not be kowtowed to?
If you would react by being abusive, you deserve no lift. End. If you would not react by being abusive then there is still no issue.
Believing, in your own head, that someone doing you a favour should be shunted out of their seat to please you is fine. We can have any beliefs we like.
Abusing her dil for not kowtowing to that belief makes the mil wrong, in every possible way.
If the mil thought her due was to boot the op out of her seat, a polite conversation could have been had, at another time. The reason mil didn't do that is she wanted to prove she is further up the pecking order in the family than the OP, and felt it was her due to demand to sit where she liked. She was incorrect.
If you are not offered someone's else's seat but a perfectly comfortable seat is offered, no normal, reasonable, decent or sane person could or would pretend that they had a right to abuse someone on that basis.
Even if you think it's rude for the wife of the person doing you a favour to remain in their own seat there is no reasonable way to demand what you think is your due without now becoming the rude person.
It's not your car. You are being done a favour. The seat being offered is comfortable. You have no mobility issues. There is no argument for saying anything to the people doing you a favour that a reasonable, well mannered person can offer.
But if someone is deliberately rude, orders a person out of their seat and then resorts to name calling and abuse when she doesn't get her own way, that is unmistakably and inarguably disugstingly rude.
The two situations are not equal. The mil is completely in the wrong, regardless of your belief about the seat arrangments.
And she's absolutely ensured she will be in the back seat now every time OP is in the car, forever. If she ever gets offered a lift again.
Don't overplay your hand in the game of oneupmanship. That's precisely what the 64 year old woman with no mobility issues attempted to do and she's learned a precious lesson. She is not more important than the OP and she'd best mind her manners if she wants a lift.
Hope she can afford Ubers in the meantime.