YADNBU
Please do not apologise this would only allow her to play the martyr. it must be made very clear to her from your DP that her behaviour was not acceptable.
Dont we all know and realise that when a small child is ill - they can either get better very very quickly OR can get worse in a flash and be danger.
There isnt time to discuss who is going with who and as someone else said - a sick child and mother - belong together. Seconds count, we have all heard of the situations particulary with menigitus where the docs have said " half an hour later and we would have lost her/him, or a leg /or an arm etc.
Its not the time to fuck around with family politics.
I think OP was very " restrained" with her MIL infact.
Op i think you did a " positive" thing, there is only so much abuse someone can take.
You might be lying to us - you might have been horrid to her in the past....or maybe she has just taken agaisnt you - she doesnt have to like you....maybe you have done soemthing to her precious son.
However - whatever you have done - she has shown a true testament of her own character in the middle of a crisis - she was more interested in passengers in the car that her own granddaughter.
how will your DD feel if she hears GM was more concerned about who was in the car than simply getting her to a doctor? What in the worst case scenario it was meningitus, what if that delay had cost her a limb?
" sorry darling, grandma was too busy arguing who was going in the car so the delay cost you your leg?"
51 is also not old.
Infact its still very very young so you cant defend her age.
I am glad your DP is looking long and hard at his mother, maybe the scales need to fall from his eyes.
I think you should be proud of finally snapping and standing up to her and you take the moral high ground, you swore - she put a childs life in danger.
I just cant belive anyone would argue differently, a swear word - and getting a child to the doctors?