The sling is only to elevate the arm to alleviate swelling. By two weeks you can probably discard the sling (or just apply it to rest the arm intermittently for comfort).

You should be doing active range of movement exercises for the fingers, and also bending and straightening your elbow to the full range if possible. Twisting the wrist/forearm will not be possible in the cast, and this movement will be stiff when you get the cast off naturally.

You should also put your shoulder through a full range of movement a couple of times a day or that can seize up too!

There is nothing sadder than seeing somebody who's been in a cast for a fractured wrist and ends up with immobile fingers, elbow and/or shoulder just becasue they've worn a sling for 6 weeks!

Get rid of the sling and use the arm for light duties as much as possible, is my advice.