Just my opinion..so take it as such
I think (again imho) that "hooking up" on a cruise ship probably isnt a good idea. Only because the likely hood of someone living close by is slimmer since they will be from all over, not just your area. Besides and no offense men, but lots of guys like the idea of a cruise ship romance with no ties to the future...is that what she wants?
just let her go and have a good time, if she meets somone its on her terms, and you wont end up having to say your sorry.
I'm absolutely positive there are lots of lovely people who have met thier spouses or significant others on cruise ships, but i still wouldnt want to put myself in line of the matchmaker.
But do have a fun cruise, no matter, your sister will have a blast!