Here's my problem, I've done 14/15 quests. I went to do "the taste of death" and realised that my brother had already entered the hall of dead in markarth and has killed the quest giver.

Is there any other way of retrieving 1 last daedric artifact?
I don't have any precious saves that would be before she was killed.
If I do the quest on another character, will that count? Or do I have to do all 15 on the 1 character?

You can obtain two artifacts in Hircine's quest, but you already finished it, so there's no other way.

It has to be 15 artifacts on one character.

Dam it lol

Yeah unfortunately this achievement can be screwed up really easily. So the lesson here is don't let siblings touch your game file