NC for this in case it is outing!
I think the distance is a red herring.
We are 3hrs from PIL, who are in 80s but still mobile. SIL is 4.5hrs from them and they visit her more, sometimes to stay, sometimes doing 2/3hr round trips from a holiday base. We have always had a good relationship, including our adult kids, who visit them alone as well as with us.
Last year they cancelled on us at short notice and on a holiday, yet were reluctant to re arrange, but saw SIL multiple times, including celebrating birthdays and Christmas with her family.
That in particular upset me, especially as we had all (including kids who took annual leave) helped them move the year before. DH niece and nephew were nowhere to be seen at that time!!
Sometimes one child is just preferred, regardless of the distance. DH is like Ops DH, more pragmatic and less upset.
The turning point for me came earlier this year, when MIL had a hissy fit at me in a restaurant for something that was very much DH’s fault (he and kids all agreed!) and carried it on in the car park.
DH excused his mum saying she is getting old, I have massively stepped away and left DH to make arrangements with them.