Blade Twisting is only effecting flat damage on the tool tip for Sinister Strike. This is inconsistent with how the talent effects Backstab which appears to benefit both the flat and percentage damage.
I noticed with 2 points in Aggression Percentage dmg for Sinister Strike is increased to 106% from 100%. With 1 point in Blade Twisting the tool tip still shows 106% (The expected outcome should be 111%). The Backstab values with the same point values reflect the appropriate Percent and flat damage change. I wasn’t able to determine if this is just a tool tip error or an underlying error in damage calculations.