If its a toss up, I say go with the party that was planned first, as it seems like you clearly knew about one of them first. Go to your mom's party and tell your dad sorry, but you already made plans for that day, but you wish him the best. I realize that your parents are selfish, but if you had input into your mom's party date, I'd attend that party. But this is just my take on it all. You're 18 now so its time to set down the rules. You're an adult so there's no reason for one parent to be communicating to you about the other. Just say "Mom, I'm not getting in between you and dad anymore. If you tell me to pass on information to him, I won't do it." And encourage your brother to do the same. Flip it around and say the same to your dad. They'll keep trying to communicate through you kids, but eventually they will give up. Your parents are engaging in this bad behavior simply because they can. If you refuse to be the middle man, the chain stops. They can stomp their feet and throw a fit, but its up to you to stay strong and not give in to their childish antics.