Paying for another kids party!!

Sooo I have two kids a three year old and a soon to be one year old. My husband and i had them young and we agreed to not having birthday parties for either of our kids till they are 5, by then we plan to be in our own home and more financially stable so we won’t be asking around for help with their parties. Welllllllll, his sister has 4 kids one of which is about to turn 2, she wants to make him a birthday party but everyone is taking stuff to organize it while they put nothing because they don’t have any money because her husband can’t find a job! I was in complete shock when I got a call to get invited to this party last week and ended up getting asked to take a piñata, candy bags, and help with the food, but it “doesn’t have to be anything extreme only what we can afford” ummm like we can but we haven’t made our kids any parties because we’re trying to get more stable! In my opinion, when you can’t, you cant! The kids won’t know any better. Since birthday 1 we’ve only gotten a small cake for our son and plan to do the same for our daughter. I just think it’s wrong that they’re asking everyone to take something to this party and they literally aren’t spending a cent. They have someone getting a moon jump, everyone so far is taking food as well, someone with drink and utensils, someone with candies to stuff the piñata, someone for cake. I’ve just never seen this! I told my husband i didn’t want to go and now of course all my in laws are mad and talking shit about me. I just feel like it would be unfair to my kids, am I wrong?