It is a sacred moment shared between couples, one they will reflect back on for many, many years. When my husband I married, we decided to elope and avoid the hassles of formal weddings. Out of consideration for our immediate family, we invited them to our simple ceremony. Unfortunately, my sister in law threw a temper tantrum because the day was not convenient for her. We ended up telling her if it was going to cause her this much stress, she probably shouldn't come.
Guests are just that, guests.
Let them know your hearts and thoughts extend only as far as their happiness. Don't interfere or push your ideas onto them. Let them arrange their event to reflect their own personalities. Stand back and appreciate the opportunity to witness their love for one another. And most of all, be grateful you were asked to be a part of their moment.