Star Trek: 10 Things You Didn’t Know About The Duras Sisters

Although they always appear as a pair, there was a distinction made in the sisters' characters.

Lursa was always the more logical and controlled of the duo; the older sibling and the decision-maker. B'Etor is even described in the script for Redemption as being the more "volatile" of the sisters and attempts advances with both Worf and Picard, the latter ending their interaction with the classic "Excellent tea. Good day ladies".

Firstborn also notes B'Etor to have a "grating squawk" when she talks. The differences continue into their costumes with Lursa the more refined of the pair while B'Etor's garb presents a more outgoing and unrestricted personality. 

One thing that does link them physically to their deceased brother and also to their ancestor as seen in Enterprise is the design of their ridged foreheads. All were designed to show the family lineage, even in the prequel series because it would be a visual way for viewers to connect the family tree.  Michael Westmore's design for their cranial ridges in Enterprise established that the patterning was a genetic trait that was passed down the male line of the family "with very little change across the centuries".

A further interesting point is that the sisters are never in a scene on their own during their time in the franchise. If one is there you can guarantee that the other is close by.