Well those who chose to be a vampire and completed the main quests of the Dawnguard DLC, later when you do all these side quests you get one that says to go to Markarth and change Cairine into a vampire... Well those who already did that and got the quest again simply go to here and kill her lol

That's what I did and for some reason I got some inheritance like the game thought she was my wife but she wasn't, anyways after you kill her go back to Castle Volkihar and speak to Feran Sadri for the next quest

You get an inheritance letter from any npc that dies. It's a way of letting the player know they've died.

Oh, we'll I guess it's happens when the person has to deal with a quest cause it's the first time that's happened to me besides when I kill my own wife lol