What kind of dog is it, out of curiousity?
In addition to the prey drive that others have posted about, it's possible that this dog was just never socialized with cats when it was younger. It's very important to socialize a young dog to other dogs, animals, and children. Otherwise, they'll end up being strange to them when they encounter them the first time.
I wouldn't bother trying to socialize the dog now, since it's already intent on chasing cats. If it's over, I would either leave your cat in the bedroom or else ask if the dog can be put away for a little while to allow the cat to come out. Do you know if the dog is crate trained? If so, maybe you could ask that they bring the crate with them, or if the dog is big and the crate not of a travelling size, maybe you could chip in to get it a crate to stay at your mom's.
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