How would you feel about writing a letter to each family member? You could think about what you want to say in a low pressure environment. Your family members could process the information without feeling pressured to respond right away. It could give them a chance to come to terms with your sexual orientation.
To me, telling everyone at Christmas is like dropping a bomb on the holiday. It may sour them on the holiday for a long time. They may feel pressured to accept just to keep peace at Christmas. OTOH, they could all start quoting scripture at you at once to make you see how wrong you are.
Maybe you should discuss this with your older brother that is supportive. If he "has your back" then you wouldn't feel like you were walking into a tribunal alone. He would know your family better than anyone here. What does your girlfriend say about this? Does she want to be a part of Christmas with your family?
"Beloved let us tolerate one another. For tolerance is of God and everyone that tolerates is born of God and knows God. He that tolerates not, knows not God for God is tolerance." 1 John 4:7,8