Please don't let an otherwise polite and good person's lack of TY notes keep you from expressing whatever goodwill you would have done otherwise. I'm not saying anyone is owed a gift, but don't withhold one out of spite!
I totally understand your ponit Slartibartfast, and agree that spite is never a good reason to withhold a gift (no matter how much I would want to...). I dont think that this is about spite though. It's about normal adult interaction, and acknowledging gifts/thoughtful gestures is just a regular part of a mature relationship
. When you give someone a gift or make a thoughtful gesture, you are putting yourself out there, offering them a part of your time and dedication (even if it only took you 5 minutes to choose, purchase and send), and acknowledgement/reciprocation of one's efforts is a very real part of relating to someone and maintaining the relationship
. In this case, Nephew and Niece-in-law have continuously been lacking in maintaining the relationship
in this area.
I see it as similar to if you always call a friend to chat, and he or she never calls you (even though she or he talks to you when you call) for no good reason (such as, he or she doesnt have long distance on their phone and so cant call you). It's a lopsided friendship, and while everyone puts up with others' quirks to some extent, it's still a lopsided friendship. Perhaps this analogy isnt completely accurate, but it's just how I see it.
So, while I may want to be a bit snarky and fish for a thank you by saying something like "Oh, I wanted to make sure you received the gift, since I didnt hear from you." (which, IMO, is actually a valid reason to call as a follow-up to sending a gift- without fishing for a thank you, of course), I say dont send a gift because Nephew and Niece-in-law are practicing lopsided relationships
and it makes the OP become in danger of being taken advantage of.