Your sister first needs to make sure that there are no health issues involved. She should take the cat into the vet for an exam (you don’t mention where she got the cat, so if his vaccination and health situation are unknown, it’s a good idea anyway).
If everything checks out physically, then she’s going to have to retrain him to use the box appropriately. She needs to start by purchasing several inexpensive litter boxes and set one up everywhere that he eliminates. They need to stay in these horribly inconvenient (at least for your sister) positions for at least several days. She should then start gradually moving them towards the ultimate position – gradually meaning moving them just a few feet and then wait 2 or 3 days. Repeat over and over until they reach where she would like him to use the box.
She should keep in mind, however, the place that she has decided she would like the litterbox to reside may not be acceptable to the cat. If it is too close to his food and water, if it is in a high traffic area, if it’s in a room that’s scary, noisy, or smelly, or any other number of reasons, he may not be able to adapt. She may need to be flexible and find a happy medium for the two of them.