I'm pretty sure they are really real hippos. I dont think Disney would want to put fake hippos in a ride and water and electicy dont mix. In Busch Gardens the hippos there are real, so I dont see why Disney wouldnt put real ones in.
As for the killing of people in Africa, the ride drivers in KS probably radio them and tell the hippos whereabouts in the water and if you've looked or read in articals on the internet there is an invisible wall in the water to keeping animals away from the car. Plus just watch them so see how much they move on the ride.
Or if you want to explain to your SIL the hippos are real just tell them to poke them with a stick to find out if they dont believe you. = ]
"So many people treat you like a kid, so you might as act like one and throw your T.V. out the hotel window."- Gerard Way
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"Tastes like someone stole my wallet, ya know?"- Gerard Way