You're not a jealous aunt, it's more that your sister feels bad that she can't do extravagant things for them. It's more that your sister needs to realize that extravagant things is what aunts are for!! We have the income to do these things because we aren't having to provide for these children every single day. It has always been an unsaid understanding in my family that if I invite my niece somewhere, that I will be paying for 100% of her activities.