Cats can be extremely sensitive to changes in their diet, setting, noises, etc. Changing a cat’s food suddenly from hard kibble to soft tuna was probably a shock to the cat’s system, on top of all the changes it’s undergone since arriving at the new home.
Purina cat chow is pretty much junk food. If you were fed mac ‘n’ chese all the time, you probably wouldn’t be all that fresh yourself! I’m not one of those cat food snobs (mine get Fancy Feast, which isn’t exactly haute cuisine itself) but if you’re going to feed the cat dry food, there are better brands that may have a better effect, digestively speaking. Nutro Max is a very good brand, as is Royal Canin and Solid Gold; the latter two are quite expensive, though. But Solid Gold is quite good for digestion.
The thing about better foods is that while they’re more expensive, in general they are much better for the cat’s system, and they won’t poop as often because more of it is utilized, nutritionally.
I am not a vet, however. If the cat’s having some issues, maybe your sister can get him or her taken to the vet to get checked.
Ooh before I forget: another tip is that if the cat seems especially nervous or is acting out (urinating/spraying, pooping outside the box, etc.), you might recommend your sister invest in something called Feliway Comfort Zone. It’s a plugin/diffuser that sprays odorless (to us) hormon that calms those little guys down. One of my cats was very anxious after she moved in and after I got the Feliway plugin, she chilled out. It doesn’t drug them or zone them out or anything… just seems to make them feel at home. Good stuff.