Hello Crabman, and welcome to cruise-chat;
that's kind of a tough question--sort of like, what will the weather be like?
What it won't be is a party cruise; but since it's the Diamond Princess that means it is a 7 day cruise, and it won't be as old as Alaska cruises have a reputation for. I would think you, your siblings and in-laws and father should all feel right at home. The Diamond Princess is a magnificent ship with enough different types of activities and venues that all of you should have a wonderful time.
Oh, and by the way, the weather weill be terrific.