You need a thief weapon for sneak attack to work

Klorox

Dang. I thought "3e sneak attack" actually meant what it said.

Zanian

'Pretty sure sneak attack only works with "light blades" (and the only weapons that would qualify for that in the ee games are thief weapons) according to the rules. Well, and crossbows/shortbows/slings, but that would be a bit weird in IWD, since here it's used as a substitute for backstab.
Although granted, I haven't actually checked if you can sneak attack with ranged weapons.