I dont want to celebrate Christmas again this year, and am wondering if I am being selfish.

About 6 years ago both my parents died. My sister and I are the only ones left ( She is 48 and I am 44). The first Christmas after that my sister had me over for Christmas. Over the past 5 years, my sister has been away on holidays for 3 of them ( new York, phoenix, Connecticut with her husband), which meant i spent Christmas alone.
I convinced myself it was fine. I would buy gifts and give them to her and her husband and my niece, before they went away.
2 years ago I decided I had enough of this and didn't celebrate Xmas. It felt liberating. Last year I celebrated it again, and it sucked. I brought out gifts to her, sat through a dinner with her new husband and their family ( who I dont really know/care for) and went home feeling empty.
Something worth mentioning, is for years I have gone out of my way to buy thoughtful gifts for her and her family. My sister on the other hand gets me stuff with very little thought e.g last I got a cutting board... WTF? I recognise the Christmas is not about gifts,that is why I dont really care for it in the first place.
I dont want to celebrate Christmas this year. Am I being selfish? I need to tell her soon, as she expects I will celebrate it.