Stop nagging him, stop reminding him about updates, if you get a text tell him what it’s about and then leave it to him.
It isn’t your remit to organise the care for your MIL, just leave it to him and his sister, it won’t take them too long to realise that it’s their turn to shoulder the worry. If he can run a company then he can sort his Mother out.
However, I don’t think that anything we say will make you change,
I was once told by a friend that if she lifted up my fringe my forehead would have ‘Doormat’ written on it, I never forgot that and I changed almost overnight from being a people pleaser to actually stopping and thinking about why it was always me that got stuck with other peoples Problems.
So I stopped, the doormat sign has long gone, I still help out, but I stop and think about whether it’s possible for the person to sort their own stuff out, and usually they are perfectly capable.