IMO, say nothing and fake a family emergency.
They can't force you to stay onboard. Airlines are the same way, they refer to it as an "open jaw" reservation, and they discourage it.
Open jaw is say having a flight that is MCO>DAL>LAX and you get off in DAL and not get on second leg in Dallas. Do this once or twice, no big deal, do it a lot and they can (and will) refuse service (fly you).
But, hey, an emergency is an emergency....your great aunt tuttie died....awwww. You got to go, your family is heartbroken.
Go to CS and explain you gotta go home at next port, you've arranged all the travel home.