I haven't read the entire thread, so maybe this has been touched on. I wouldn't necessarily blame the parents. You know how kids are; the parents might be hounding them to send a thank you note, but the kids don't do it. I know that as a parent myself, I have over the years had to do it myself, and just get the kid to sign their name, but that gets old pretty quick. It doesn't come naturally for them to send a thank you note immediately. I do, sometimes, have them call their grandparent to thank them. I hope that counts. It's just one more thing for me to have to manage, and honestly, I'm tapped out. I think this piece resonated with me
http://www.scarymommy.com/never-write-me-a-thank-you-note/
Also, as a caring and loving aunt, you could do some persuasive "parenting", too. I will never forget getting an awesome gift one year from a grandparent. I loved it SO much, was so excited, I started to ask all sorts of questions, "Where did you get it?" "How much did it cost?" I was promptly put in my place and told that it wasn't polite to ask how much a gift cost. It had never occurred to me before that it wasn't a question to ask. Sometimes the best message a kid can get is one that comes from someone other than the parent.