Ok - I want to bring a bit of a different perspective here. I was "that sister" for my parents 50th anniversary party.
Yes, I work and make a what people would consider a rather good salary. But I am also a single mother. I adopted as a single parent, so no child support or anything else like that. Just my salary to live on, maintain an emergency fund, pay for home repairs, save for retirement, save for daughter's college, etc. My daughter is in a private school because she has a few issues that they can address better, and so I have to pay tuition for that too. Quite frankly, even though I have a good career and make a good salary, I struggle to keep a balanced budget (and no, I don't have any debt other than my mortgage and a small car payment). Right now, my savings rate is nowhere near what it needs to be and I have to take responsibility for that first and foremost, to be able to take care of my daughter if anything happens. So, no, I wasn't going to spend a bunch of money I know I couldn't really afford on a 50th anniversary party.
Thankfully, my siblings understood that my contributing just wasn't an option - without needing to know the details, and despite the fact that I have a better job than some of my other siblings without kids, and send my child to a private school. They never asked me to contribute because they sensed I just couldn't - and they never, ever asked me to justify why I couldn't or challenged me in that regard. And they DID put my name on the invite. I helped out where I could with details like the seating chart and placecards, took care of getting my parents to and from the events (we had a whole weekend of stuff), etc. - I was there to support my parents, and that's what was important to everyone, not how much money I was able to put in.
Just another perspective to consider. You may think you know your siblings finance's well enough to say they "should be able to contribute". But how much do you really know?