YTA.

The fact that you skipped hosting so many dinners means that you can use those funds that you would have budgeted for those meals to instead buy enough food as required for the whole family. I'm sorry that money is so tight for you that you can't afford to feed two children that have voracious appetites, but considering you have saved so much money by not hosting and saved more money by actually eating free dinners from others who have hosted, you should be able to host twice a year. I understand how spending an extra $20 (as your SIL has stated she does) just might not be feasible for you and your families limited budget, but you could always do spaghetti, salad and garlic bread and make triple the amount so that everyone is able to eat as much as they want. If others complain about it being a cheap meal, explain your circumstances about not being able to afford it (as that's really what it comes down to). Don't food shame your niece/nephew because it annoys you that your whole family can eat the same amount of food that the two of them eat. That's just being mean.