Imperator Siffow's original post indicates that your grimoire bug is currently in Bungie's radar as an issue to fix and Siffow does not need any new reports on its existence or per-player impact. I am not saying this to be dismissive of your situation. I am saying this because the thread you are posting to is intended for the reporting of grimoire bugs [b]not[/b] on Imperator Siffow's list. The flood of posts indicating known bugs drowns out the posts of bugs not yet known to Imperator Siffow, ultimately making this bug reporting mechanism less effective. That said, others have had success acquiring these two cards by killing the sisters in the story mission where you return to Crota's realm to steal his soul while invisible. Reading some of the below posts for specific information on this strategy could prove helpful to your situation. From what I know, you need to be in the Gatekeeper's pool room no-scope sniping the Deathsinger Sisters before you go to Crota's platform to steal his soul. You may need to have a full fireteam focussing on one killing one Deathsinger, and then the other. They tend to see you after so many sniper shots, even when no scoping from the aforementioned room.