Report Leifnarr's death to his family is a miscellaneous bugged quest, so it cant be completed. There are two possible solutions.
What to Do to Actually Complet the Mission
You must talk to Grosta, Leifnarr's wife, before finding his dead body, so mission is activated before that happens.
If it is too later, write after pressing "~" (to open console) SetStage E3EA5 30, it is important that you press spacebar between E3EA5 and 30. This cheat will make you able to talk Grosta about his husband death, it does not complete the mission. It fix the bug completely!
Where to Find Leifnarr's Family
You can find Leifnarr's family here, in the samwill located at the west of Riften.
I hope this helped you, this bug is very common, and many people had complained about that.