I'm sorry if this is long winded but I mean well Op;
Op, you need to stop comparing what your pil do with their dd's children in comparison to your dc otherwise it will eat you up and have a negative impact on your marriage.
You mentioned that you haven't spoken to you dh about it, you are just running it over and over in your head, driving yourself mad and becoming more resentful, that is not good for your mental health.
Every three weeks is good even if it is only for 30 mins, believe me. Maybe your pil need to be asked and are trying not to thread on your toes too much?
It seems to be a problem with il's for most people are: either the il's drive them mad by being around too much/not enough or spending too much time with one lot of gc over another lot.
It's not easy but for you/your dh/your marriage's sake try to be fair about it whatever way you do it. The main thing is that everyone is in good health, in good form and around to spend time together especially at Christmas. Your dc is lucky to have all gp alive and well and can get to know them, not everyone is so lucky.
Snubbing your pil just to get them back for not spending equal amounts of time with all their gc, will only cause a rift. When your dc is older, it would be nice for them to have nice memories of both sets of grandparents spending time with them, it is quality over quantity. Your dc will lose out the most if you start excluding your pil, especially if your pil are not even aware of what the problem is.
It is not always easy but do your best :-)