Stefan and Damon's mother was a human and she won't be coming back....that's for sure

Well looks like Annie who plays Lily, everyone has her rumored to be Mrs. S. So here's hoping. It's about time, it's long verdue, glad JP put her in.

Could be a flasback scene.

The questions now are: Is she a supernatural being and why is she in one of the Gemini covens prison worlds?

I'm guessing that Mary Salvatore found out about a coven..threatened to tell someone then was banished to another prison world. However I do feel as if this somehow connects to Kai? Somehow Bonnie was transported into a completely different prision world which i don't understand at all

Kai is the key to the prisons. It's like how Bonnie was the archor for the two dimensions (Living & Non). Kai became sick b/c of his foul merge with the wrong twin and that apparently caused the prisons to start deteriorating and collapsing ontop of each other. 

My guess at the circumstances is this: Stefan and Damon's father found out that their mother was involved in magic. She was either a witch or vampire, maybe some other creature. He made an arrangement with the coven to imprison her b/c of his distaste of "evil" magical creatures, probably telling them that she was all powerful or giving some other reason why she should be jailed, and then told his children that she had become ill and passed away because of it. 

It'll be interesting to see what the Salvatore's Mother really is, supernatural or human.  For now, I'm going with human and actually someone that may not have had any clue about where they weere at.  Bonnie found that journal in the house and if you paused or looked at it quick enough, what was written in it gave the interpretation of someone writing a diary and keeping track of how things worked there (probably were Stefan picked up his need to diary his life).

Well, Kai told Damon that his mother faked her death because she became a vampire. She was a "ripper" and wiped towns out. Soon the Gemini coven decided to put her in a prison world made just for her. Seeing how Damon was really upset when hearing that, I don't know if they'll bring Mom Salvatore back. But there should be a high chance. Because what's the point in showing her face on the tv show and then say "oh she's a ripper, we can't take her out of her prison world" and then drop that subject. I think Julie is going to make her come into the show.

Well guess again lol

well well well...