What ability allows sugar molecules to pass through a visking tube?
Are you sure that sucrose can pass thru a Visking tube?
Sucrose is a fairly large molecule, and (as far as I know) is too large to pass the Visking membrane. And thus a Visking tube filled with sucrose solution will become distended in that the osmotic pressure favours passage of water into the tube, i.e. towards the higher concentration.
Anyway, I am no biochemist, so check what I say in your text......