Valygar cannot backstab with his personal weapon even though Katanas can normally backstab.
Corthala Family Blade can inflict backstab damage.
Just tested this out because my brother brought it up to me. What's the purpose of making a guy a kit that can backstab then giving him a weapon which can't do it? I think the error lies in it's not usable by "Thieves" which is the only value the game looks at when factoring in backstab damage. As the sword is only usable by Valygar, it is automatically disqualified.
BG2:EE is far down the road still, but I thought I'd bring it up. Are there any mods that fix this?