Anyway, there are one or two very rude and demanding people on this thread - but fortunately vastly in the minority. Most of us do understand the situation and would have used our manners and been thankful for the lift without throwing a tantrum demanding OP move.
No point in continuing to repeat the obvious to rude people pretending not to get it, so I will bow out here :)
A reminder, if you are being given a lift it is the height of bad manners to make any demands of your host at all. You sit in the available seat. That is the rule. And that is good manners.
If you imagine yourself more important than your host or their family, you are a rude person. If you do not then you would not be expecting to be treated as more important by usurping someone from their chosen seat.
So that's that:)
If you have accessibility issues, that's quite another story of course. And not relevant to this particular thread.
But in a situation where all seats offered are similarly comfortable, and you are being given the favour of being driven somewhere, only the most repulsively ill mannered, entitled creature would expect preferential treatment by demanding their seat of choice. And nobody at all would ever be expected to give up their chosen seat, for no apparent reason. That's not a rule at all, of course.
Once again, the only reason the mil was so viciously ill tempered and ill mannered was that she planned to usurp the OP in the family pecking order. That's the entire reason for her tantrum.
She failed and has now learned a valuable lesson - she does not rank more highly than the OP in her son's eyes and she cannot speak to OP in such a disgustingly offensive manner and still receive the favour of being driven around.
I will leave the squabbling, ill mannered minority to keep throwing little tantrums. I have many, far more important and rewarding, things to do.
As I am well raised and very polite, I give you my permission to have the last word. It will make you feel better about losing the debate :)