I think you just have to tell them. Just be brave, make the phonecall and be honest. There's no point in making up excuses that aren't true because you'll just tie yourself in knots and be found out in the end anyway. Honesty is best. You might have to hurt your family, but they'll get over it. And you only have to have this talk with them for the first time once - it'll never be this difficult again, because next year they'll be over it and they'll be expecting you to do the same because you've made that break with tradition this year.
You can do it kindly, and with grace and respect. Good luck, and I hope it goes very smoothly. Chances are that your family will be very understanding, you never know.
family is always important, but you are on your right to choose to do something different and new with your kids. Just let your family know your plans, but never forget to let them feel special and loved.....
As a parent of 4 adult kids who have used that theory on me at least once each, I would rather have them tell me the truth instead of snowball me. To their credit they always tell me the truth. How do I know? I grill the grandkids LOL works for me.