"I do make a big effort and feel resentful that BIL doesn't help"
He does, in a way he knows matters (them being his parents).
My Dsis probably underestimates what I do for my Mum and how I rearrange my time.
It probably does look like I an going places, or have nothing else to do, when I am with my Mum, but in truth I am spending the time that I do with her, because it keeps her going, both physically and emotionally.
Once a week, she goes out shopping independently and gets a taxi home, I give her a few items to pick up for me, because I know it makes her feel useful, still.
I don't resent it, but it is a strain.
I think you are underestimating what your BIL does, tbh.
You have decided these get together a are important for your PIL, so you arrange them, you can ask for help, but when the sons of your PIL say that they don't think you should bother, either do it yourself (you can have words with your DH) but you have no right to dictate to your BIL.
Tell your DH that he has to have more input into his parents lives/happiness. Take it from there, your BIL probably keeps them going and you put the icing on the cake.
I couldn't host a party, we go to my Mums or eldest DD's, my house is to small, I don't have a kitchen table, because of my dog.
As said, he may not cook to his patents satisfaction.
Your MIL may not like the idea of her son having to cook, he helps her still feel useful (probably) you may find it all backfires if you carry on the route you want to take.