This thread has made me really sad.
OP you've done a lovely thing 
I can't understand why people are acting like her sister and nephews are getting nothing?? The children will still be getting their normal presents from their parents, grandparents and any other aunts and uncles, won't they?? The OP hasn't cancelled Christmas all together!!
If my brother decided to donate instead of buying gifts, my children would still be getting presents, from us/Santa, my parents and DH's parents..
We stopped buying for the Bruce's and nephews on DH's side a few years ago. One, because there's 13 of them now and we just can't afford it, and two, the last time we did do presents the comments from two of the nephews were "is that it?" and "where's the rest?" 
We only do birthday presents if there's a birthday party.
This is across the board by the way, not just us not buying for anyone, it's what we all do now!
I hope you have a lovely Christmas with your DS, and that the new year is kinder to your loved ones 