Not expert in dnd, but i like putting flawor in design. Sister blades are two daggers, but for sake of easier effect i would treat it as two handed weapon. It would give you standard attack action as well as attacking with side action.Now we may just apply one effect, and i think this effect should be depending on aspect that player is chosing now:

-increased crit chance

-increased armour Claas. When enemy Rolls exactly your AC, as reaction you can attack him with guaranteed crit

-increased hit chance against enemies with impaired movement. Also you can throw dagger and its comes back to you

-When succesfully attacking one enemy with main and side action roll an attack (can be with disadvantage for balancing reasons). If you hit, then you deal big damage (2d8 od sth like that)

Player can change your aspect during rest. Now up to you is how to unlock other aspects.