Get them to perch up on your compost bin and take a shit into there. Free manure. Good for the garden.

Explain the situation, they'll understand

that most have been some shitty party.. no? ok.

Rent a portapotty and blame the water being off on a plumbers mistake.

Oh that's all right, op. There's no shame in saying your water temporarily not running. Happy birthday !

Go to bowling, pool hall, park, etc. Some places let you hold a party there for free, just tell them you dont want to pay for any games just use their space. They don't usually charge for that because you are bringing a group that most likely will pay to play anyways.

You can rent a port a john and get some antibacterial...or have your party somewhere else. Problem solved. Enjoy your party.

Tell your friends that your sister blocked them all. :P

Oh, you need an excuse to tell people not to take dumps at your house? I always tell my guests at the front door to take their shoes off and they're not allowed to shit in my house. I don't get much company . . .

make #2s in a garbage bag around the toilet lid, dispose afterward