It's a legality issue I believe, they're officially on the black list and if you are seen with a wild caught boxie it can be a very hefty fine.... It's cute, yes, but they're endangered in the wild due to the exact thing your sister did (a bigger scale if course, but you get the point ).
Because the number of box turtles in the wild is decreasing dramatically - largely due to people taking them as captive pets.
They will also fair much better and be healthier if left in the wild.
The only reason who would keep it (temporarily) if it was injured is if it was an injury dramatic enough to require intervention to save the animals life. It is very wrong to keep an animal from the wild as a pet, and everyone will recommend putting it back.
It is also specifically illegal in many states to take wild box turtles, and you could get a very hefty fine for it, depending on where you live. He belongs back where your sister found him, there are plenty of other ways to get them as pets.
Box turtles make excellent pets. But they are so rare in the wild that it is a tragedy to take one from the wild. It is easy to buy a perfectly healthy boxie from a knowledgable breeder. That way you get a parasite free animal that is acclimated to a captive life. Also babies are so difficult to raise its probably better to start with a sub-adult ;-)