Don't some cruise ships offer babysitting?

"forced"? sorry, not true. you could have said no. even if they snuck over there leaving you with them, you could tell the staff that they're not watching their kids. you are never "forced" to watch another person's kids.

Why did you pay for yourself and 7 other people to go on a cruise?? Don't they have some kind of day care on that boat?

55 - what makes you think they didn't leave without telling OP and stuck OP with the kids? Hence "forced" as OP said.

Damn, bummer

Is there a little camp on board they can go on? I was on those when I was on a week long cruise...

Who said any of them are his?

Tell security that five kids were left unattended, and that you believe that their parents are in the kid free zone. Security will take care of the kids, find the parents, and your sister and bro in law will learn the rules of parenting real quick. You've got to learn to do what is best for you. Your sister made her own decisions. She'll deal with those consequences. You don't have to be a suck up and pick up the pieces when parenting gets rough for your sister and for Mr. Deadbeat-Dad. Handle your business and enjoy your vacation. You paid enough for this. Cruises aren't cheap. Life is what you make it. Good luck and have fun!

5 nieces? You should of told your sister to keep her legs closed

I'm slightly confused why you bought your sister and her " deadbeat boyfriend" tickets in the first place...