Instead of avoiding this family gathering, why not ask God to help you be a peacemaker in your family? Jesus said, "Blessed are the peacemakers, for they will be called sons of God" (Matthew 5:9).
How can this happen? First, ask God to help you change your own attitude, and to give you a deeper love for those in your family–even those who aren't easy to love. Remember, God loves them (even if they don't love Him), and He wants you to love them also. The Bible says, "This is the message you heard from the beginning: We should love one another" (1 John 3:11).
Then pray for this gathering–not just that it will be harmonious, but also that it will be a time of true thanksgiving. Even if it hasn't been your custom, begin your time together with a prayer of thanksgiving. The Bible says, "Give thanks in all circumstances, for this is God's will for you in Christ Jesus" (1 Thessalonians 5:18).
Finally, ask God to help you be a witness for Christ to your family. Elsewhere in your letter you mention that there aren't many committed Christians in your family. Share your concern with those who are, however, and together make it your goal to show Christ's love to your family. The Bible says, "Make every effort to live in peace with all men" (Hebrews 12:14).