Yup, us too.
My BIL's two children are the favourites. Our children are not.
MIL gives regular overnight and daytime childcare to my DNs. SIL's mother does the same. SIL is a SAHM but has regular support from either side on 5 out of 7 days of the week. (Plus emergencies.)
Meanwhile PILs have not seen my kids for 5 months. FIL has only seen my 9 month old twice ever. They say that we don't need help (we both work full time) because we handle everything ourselves. Yes, we do, but it doesn't mean we don't want them to visit or show interest. We have to be organised with childcare because we work, but it is very very hard. We don't expect free childcare so we have taken care of it, but that doesn't exclude parents from helping if they want.
The bit that really upsets DH is not so much the help - that bit could be marked down to proximity and circumstances and PIL not appreciating that giving one child money/practical help and not the other feels unfair.
It's stuff like MIL not being aware that our 9mo can stand up, crawl etc and she keeps telling how DN1 is a genius. If my DS1 (aged three) has started to spell out words, then so can DN1, even though he can't walk or speak yet. She never texts DH except to reply to his texts telling us what DN1 has done with no questions about our DC.
I wasn't so bothered until MIL spent several hours telling my own DM about DNs achievements. She didn't even focus on the grandkids they share as "she doesn't see them much". My DM was stunned as before the DNs were born, her and MIL chatted lots about our kids.
The worst thing is we get on well, I like my IL and I thought they liked me. My DCs love them so much. SIL and BIL are always moaning about how much the IL annoy them and I find it so hurtful that I try so hard to have a good relationship, encourage them to see GC and yet my kids are still second best.
Rant over. No advice sorry. I veer between wanting DH to say something because otherwise it will fester and letting it go past and hoping it will improve when all the DC are not so small.