Let's pretend you are an axe and mace specialized warrior.

When you wield an axe in your main hand and a mace in your offhand: You can whirl, hook, weaponbreak, etc. but you cannot backhand, boneshatter, dent, etc. because your mace is in your offhand.

When you have sisters. That changes. Now you can use your spec skills from the offhand.

One caveat is that it only works with spec skills that require a command. You cannot go sword/dagger and expect to flourintine AND concealed.

With fairly recent changes (last year or two?), sword/dagger is actually at terrible pick for it. Once upon a time, you parried with your mainhand, so it would make since to have sisters, wield a sword for parry, and use dagger in the offhand to bleed them out. Now, if I understand correctly, your parry has a 50/50 chance of being attempted by your primary or secondary weapon.

Essentially, for sisters builds, you're looking at using mace/axe/whip/dagger combos. I would say. I'm by no means a professional warrior, but I'm confident I've explained the legacy accurately.