How to respond when people don't say thank you?

I have two nieces who rarely thank me when I give them a present and am not there to give it to them personally (e.g., I mail a gift). No call, email, text, nada.

I faithfully gave them presents until they turned 18 and then stopped for three reasons:
1) They never thanked me.
2) When one had a birthday, I would send a present to both of them and one day when it came up they didn’t even acknowledge that I had done so and when I brought it up they were silent – again, no thank you, no “that was cool”, no nothing!
3) During the 24 and 29 years I have known them I have only received one birthday card from one of them.

I decided two years ago to give them a second chance because they were older and maybe wiser (?), but did not receive a thank you after sending gifts and so have decided to stop again. I don’t do it for the thank you, but I guess I feel that if they don’t appreciate the gift, I would rather spend my money on someone who does (like myself!!). What puzzles me most is that these girls are not wealthy. They struggle to make ends meet – why wouldn’t they say thank you for $100?

Here’s my question – how do you feel about my decision not to send presents going forward? I have two other nieces who always thank me for their gifts and as I am thinking about my will, I don’t feel like splitting the money equally.

I feel guilty about this because I don’t like being unfair, but I guess I feel just as guilty about rewarding bad behavior.