If one of Oryx's sister's sought the 'pinnacle' of sword logic, it is likely that they will come after the PC Guardian in the future. After KF, the chain works like this: Guardian>Oryx (who is now dead)>Xivu Arath/Savathûn. The Guardian is the new pinnacle. Either these sisters intend to take the pinnacle for themselves or they want a warrior of light to be the strongest (for some inexplicable reason). I don't think Bungie puts this much thought into their story, though. Maybe we're overthinking it.