Ledkr, we have a similar thing here.
We are all now local, but this wasn't the case in the past.
GPs do regular and adhoc childcare for other gcs. They offer to have mine whenever we speak, but 9 times out of 10 it falls through (from their end). And whenever they are there, the other GCs all suddenly appear "to play".
My eldest nephew happily tells my dcs all about how he does special things with GPs all the time. I don't think there's been a time when they've been there without their cousins in the past year.
So my dcs visit maybe once or twice every couple of months for a few hours. Other GC are looked after maybe 5 times a week.
I wouldn't mind so much, but when an old family friend came to visit and said to me, "Your DM looks exhausted. She does an awful lot of looking after all her dgcs, doesn't she? You should give her a break." I was open mouthed I'm not sure I replied.
Or when I asked if she could babysit so that dh could attend an important hospital appointment with me, and DM said, "I've got nothing in the diary, but I'll have to check that your dbro doesn't want me to look after his dcs, before I can say yes."
I'm grateful for the help I do get. But devastated by the two tier system in place.
So start as you mean to go on. Because it is hurtful when your dcs are treated so differently.