I'm with those that think you're overthinking it! Your reasons for asking the family to come are your own and you don't need to explain everything to them even. It truly is as simple as saying, we're going on a cruise - we love it, would love for you to join us. What do you think?
I have gone on vacations with family and friends - Disney Cruises,
Disneyland, Disneyworld, Disney Paris, Hawaii, skiing, camping, etc.. I love vacations when it is just our family and I also love those vacations that we share with people in our lives. That's what life is after all? Building relationships and making memories with family and friends?
Because I am a planner, I am usually the one that starts the planning and makes bookings first. And I have no qualms about asking friends or family to join us. It is completely up to them to go if they want, the vacation time works, and they can afford it. I don't pressure (well maybe I do by telling them how fun it would be!). But I really don't see how it's presumptuous at all when you are just
asking "Hey would you like to join us? I think it would be fun!" If they can't afford it, they say no. If they don't want to come, they say no. If they do come - it makes for priceless memories! My son still talks about the night he rode the Aquaduck 28 times with his friend who came on a cruise with us ( he came with his whole family!).