I have killed both parties in their respective residences and took the ceramonial dagger and the family ring and placed them next to the deceased people, but I cant seem to get the "quest complete"...Am I forgetting something? Thanks

Don't you have to go outside and talk to any member of either of the two families? After you've done both deeds, and you have, go outside, wait til morning if you need to, or simply go talk to the woman at the inn/tavern there. She sleeps behind the bar, iirc. Then everyone freaks out, but do not join in the fun.

For some reason, I'm still not getting the quest completed sign. After killing both and planting the evidence, I've spoken to family members and the bartender and there is no excitement by anyone? I returned to the shrine, still no response.

Here's the http://www.uesp.net/wiki/Oblivion:Mephala. Look at the bug section. Planting evidence too fast... idk, but go back and replant it it says...

You are to place the evidence (the ceremonial dagger and family ring) in their inventories not "next to the deceased people." Once you do that, you should get a quest update.