I wish I knew how to make it okay! My sister is the same way with my kids- teens, now, for Christmas... nothing - no card, etc... same thing happened for my older son's graduation in June. She is on a tight budget but at least a card or phone call would be nice. But then when she tells me she is shopping for a gift for a friend - grrr. I just send my nephews gifts. Maybe one day she'll get it!
), but, when she called him back the next day, he wouldn't answer. we've both called many times since, and he doesn't answer (he's retired and doesn't get out much, due to my step-mom's health, so he IS at home).
) BUT they went to the extreme of no longer acknowledging their grandson and his daughter.
to "Oh crap, is it the 18th today. Thanks for the reminder." She loves all of her grandchildren very much but they live 1800 miles away and easily get distracted. The card/gift was sent the week prior to arrive in the mail on time but that phone call sometimes slips. Heck, she gave birth to me and my phone call sometimes arrives a day or two later. 
Really, no joke.