had the same yesterday, and can say its a bit bugged, but u can still complete the quest.
i had bought the house before i finished the quest. and becouse i didnt know the location yet i tried to search....
infact all u need to do is talk with:...
Spoiler
- the guard at the graves
- the woman inside deathhall (the door infront of the guard)
- go to jarl and ask the guy (where u buy the house) for support.... (this did not work for me at that point)
- from here i ran to all npc's again.....nothing happend xD
- finnally i found the house (crimescene).... when entering and looking at a chest, a quest updated.
- go back to the old woman in that death hall.
- from here follow the woman back to the house.
- enter it follow the steps.
- go to the house buying person at the jarl.
- ask support from him
- from here on the quest works as intended and i finished it.
.... The only problem is, i got a nice house now, but all the blood and the ritual things are still there, even after upgrading the house so u have all the stuff

true quest is a bit broken, but u can still finish it.