To be hurt by how little my parents want to see me and my kids?

It's understandable that you are hurt. It's also understandable that your parents struggle to fit in many visits with their current situation. Could you talk to them about how you feel? You could maybe talk about ways that their load could be lightened so they have more time for you.
They may not realise how this is making you feel. How often did you see them before having DC?

Clearly you have never been in their position to work and be a full time carer as well.

I would never ask a child with young children to help me clean my house. They have enough to do.

I don't see how anyone could possibly think you're being unreasonable. Obviously you understand their pressures with work and caring for your sister, but for them to prioritise housework and seeing friends over and above their own daughter and grandchildren is very hurtful. I can't imagine prioritising socialising with friends over seeing my own kids and grandkids. And yes housework has to be done, but why not ask you to help? If you and DH took the kids over in the morning, one of you could watch them while the other helped with housework and chores and then all spend the afternoon together. Where there's a will there's a way.

I'm sorry they are a let down, OP. It would hurt me too.

A 2 year old and a 6 year old are usually exhausting together and lively and if boys bouncing off the walls, they are probably exhausted from work and your company will be full-on, regardless of how much they are being watched. It’s sad but some grandparents are just not that interested in constant interaction or involvement, yet others throw themselves into it like a job. We’ve got one set of each and neither will change it’s just how they want it to be. I would keep visits short in case they put them off if they think it takes up all day etc

I'm sure they want to be. They are caring for a disabled child (severely by the sound of it) and working full time. They aren't superman or woman.

Will his parents not do anything, either? From your post I guess not, as you say you're (understandably) sad your kid won't have grandparents involved.
I would make a point of speaking to them-maybe invite them out for a coffee just with you. Sometimes in-laws can be scared of seeming pushy, and believe their DIL won't welcome them, and that they shouldn't ask their son. This is stupid, but sadly common. If you make a point of telling them how much you want them involved, maybe it'll make a difference, as it seems your parents just don't want to be from what you say, and I'm sorry.

It sounds sad but they seem overwhelmed dealing with everything else they have going on. Plus they aren’t getting younger. I think a lot of us can be guilty of expecting our parents to be there when really they need rest too.

Working full time and caring for someone with severe mental health problems. That will explain it. They may be running on empty. Caring is exhausting enough without working full time too. I'm sure they'd love to visit more but know their limitations.

They work full time and have their hands full. YABU

They're probably just knackered.

Sounds like they have a lot on their plate, both working full time and supporting their daughter with severe mental health illness. I wouldn't cancel Christmas plans, I would make the most of the times they are able to see you and the children.

Sorry, they both work full time and are supporting someone with severe MH issues?

yabu

sister's severe mental health problems.

Sounds like a lot to deal with

Did your parents actually want to be grandparents though? It's not anything they have a choice in happening but they do get the choice on how much contact to have

It IS sad and also annoying that you have to do all the work. Could you bring it up with them? Just tell them honestly how you feel?

You feel how you feel.
But if they’re say 60ish and still working full time, they’ll be knackered come a weekend, and have all the chores to do, their friends, their hobbies. I don’t think once in 3 months is particularly low given that. If you want to go out with your husband use friends as sitters, use a paid for babysitter or what we mostly did was had dinner parties where everyone including the kids stayed over.

It sounds like they are completely overwhelmed with dealing with your sister and have very little left to give. Sad for you all though.