Op - you are totally justified, in fact I cant believe that you have kept on so long when they have ignored you!
I have had similar with my DB - any perceived slight to him resulted in nothing to my 2 for xmas and birthdays for a year or 2, then he would re-start buying for a while until he decided to stop again.
I always kept buying my niece, as it isn't her fault her dad is a twat and I wanted to keep the moral high ground. Plus wanted to keep the peace with my mum, who would always side with him, the golden child, even though she knew he wasn't buying!!
However, after a massive fall out 2 yrs ago, he immediately stopped buying mine and I know will never restart. He even had the audacity to remind me of his DDs birthday last year. I had already got her card and a cheque ready to post so sent as normal, but decided there and then to stop.
He did drop some hints before DN's recent 21st but I ignored, after all he ignored DS's 18th.
I am not usually petty and don't buy to receive but there comes a point when it feels they are making an idiot of you!
Send cards if you want (we dont even do that now) but stop sending presents. Do as your brother does. If your Aunt questions it, just say your DC has received nothing in return for years. Be honest.