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.