Eauchiche....I would continue to send the money, just as you have. You have no way of knowing what it means in their hearts.
In the pure sense..a gift comes from the heart because you want to give it...not, predicated on getting something back....If you give...just to get something back (even a thank you card)...then it is more of a manipulation than a true gift....
for myself...I can remember getting gifts from aunts, as a child that I didn't send a thank you card to...and the gifts meant the world to me! (I wasn't taught , as a small child to send thank you cards to extended family. I would say "thank you", in person, if they handed me something....and, maybe, my mother might have thanked them, beyond my range of hearing---I just don't know.
If you give to one kid, and not the sibling...it can foster feelings of sibling rivalry and all kinds of bad feelings...
At least, these are my feelings on the subject.....