Why is dialysis tubing impermeable to sucrose?
Benedict's reagent detects reducing sugars (free aldehyde group) by reducing soluble blue Cu(II) to insoluble red Cu(I) oxide.  Sucrose aquoous solution is inert unless it is pre-hydrolyzed by heating with strong acid catalyst.

That certainly is a poser.  One possibility (SWAG, not fact) is regenerated crosslinked cellulose is overall not permeable to neutral solutes.  Its size selection is active toward dissociated solutes and proteins that are charged.  One can test this with anti-freeze.  The sodium fluorescene color should freely diffuse.  Tiny molecule ethylene glycol (EG) should be retained.

https://www.chemetrics.com/Glycol 
  Test kit and how it works:  Iodate cleavage  of 1,2-diol to aldehyde, then aldehyde detection with Purpald. 



One could iodate cleave permeated EG (or propylene glycol, glycerin, etc.) then use Benedict's solution.  Serial dilution of standard EG solution for control and calibration.  One expects any dialysis membrane to leak somewhat.  Major passage of uncharged 1,2-dihydroxylic solutes smaller and larger than than sucrose is the SWAG's falsifying observation.

http://chemsee-foods.com/antifreeze-detection 
http://www.chemworld.com/Ethylene-Glycol-Test-Kit-p/k-4815.htm 
http://www.hach.com/glycol-visual-test-kit-model-eg-1/product?id=7640218461 
  EG in food and fluids tests.   
http://www.astm.org/Standards/E202.htm 
  Testing standards