Just say no thank you. You don't need to lie and say 'birthday party' or whatever. It doesn't matter what sis thinks anyway. If you don't like him, don't bother with him. Or, as the lovely berries said, he'll just keep calling and you'll keep coming up with excuses and avoiding the phone... The games get old quick
If you can't tell if it's a date or not but you are afraid it is. Bring Dennis with you when you go. That's give a permanent message that you are not interested.
I'll be in Cacun next weekend. Rice, just tell him that you thought about it and would rather not go out with him. As everyone has said, it's better to be honest than to avoid him. If you're honest, he'll go away. If you advoid him, he'll keep trying. Jamie