My sisters dog had round worm, what is that? How contagious is it? And what r the symptoms?

Worms. Common and are contagious through handling dog excrement. Take your dog to the vet, and get proper treatment for roundworms. Keep children away from, properly collect and bag, then carefully wash your hands (using disposable gloves with an inverted bag is good for disposal).
Yes. It is possible to get roundworm infection from your dog, depending on which roundworm it has. The best advice is to get the animal treated and use precautions like hand-washing when touching anything which may have been contaminated by its feces.