OK, here are a couple of ways you can try and convince your brother and sister-in-law to venture away from a
Disney cruise...you know, just in case you find one on another cruise line that you feel you might want to do!
We have also looked into the Disney cruises. They are much more expensive than other cruise lines like
Royal Caribbean, Carnival, Norwegian, etc. That's why we don't go on a Disney cruise. Of course, I am also a very experienced cruiser (this will be my 14th!), and I am familiar with these different cruise lines. I understand that there are many people who feel more comfortable going with Disney because they are very familiar with Disney, and they know it will be spectacular because it's Disney!
We are going on Royal Caribbean. Our ship has a rock climbing wall, an ice skating rink (with shows there!!), a mini golf course, a 24-hour promenade inside (shops, a cafe, bars...food in the cafe is included and served 24 hours!), parades some nights in the promenade, lots of things to do.
I also don't understand why Disney doesn't offer the 7-night Caribbean itinerary in the summer. Maybe they figure that people will want to do a shorter 3- or 4-night cruise and spend a few days on land also, touring the theme parks. They always advertise land/sea vacations on their website. If someone only has a week to go on vacation, they could do a 3- or 4-night cruise plus 3 or 4 nights on land and have the best of both worlds. Maybe they feel this is more in demand in the summer since kids are out of school.
I agree about the summer...the air is very thick. That is the main thing...not the heat, but the humidity. And of course, I forgot to mention the afternoon thunderstorms. Another good reason to leave the park at around 1pm! Of course we got caught in one of those on the way back into MK since we had an early dinner ADR at CRT, and it was about 4:00 when we got off the bus, and it started pouring! We went there for dinner completely soaked

(ponchos didn't help much because of the wind), but we all enjoyed it! Good point also about how late the parks are open in the summer. That has not affected us yet since the kids are little, but when they are teenagers I am sure we will be taking advantage of that!