I wonder if you're running into their assumption you don't need any help because your DH is working at home. So even though you know your DH can't help, they may think that he can help, so you don't need them e.g., DH can watch the kids while you go to the dentist. They may think your siblings have to go to work, so that is a higher priority than you volunteering at DC's school (not saying it is a higher priority, but they know the other sibling can get fired if not at work) Plus, on the days they're not helping the other families, they want time to do their own things. Do they offer to help the other children or were they asked to do so? They may also have offered at one point and now it's become too much, so they're not looking to add any extra child care to their plates, especially if your DC are younger so already had these commitments to the other siblings. Do they know you want more help? They may think you've got everything covered and don't need any help. I'll be more annoyed at the siblings to be honest if you feel they're taking advantage of your parents. Could you approach your parents about spending more time with your DC? They may be a better way to address rather than "they get childcare and we don't."
FIL retired early for health reasons and SIL wanted him for childcare on days she was a substitute teacher. He did one day a week. SIL wanted him to do more, and MIL put her foot down as that one day a week was very exhausting for FIL. MIL was still working, so she couldn't help out during the week. No other grandchildren live close by. MIL has flown out to look after DS or we've take DS to MIL/FIL so we could have a weekend away. Anytime MIL visited, DH and I would have date nights. FIL's health has gotten much worse, so she can't leave him anymore. My sister now lives in the USA, so we pay her to fly to us to look after DS, and if she's visiting we take advantage of her time and go out for a couple of dates. We've always paid for babysitters or swap time with other families - sometimes it's just easier to pay someone than deal with family.