だれが？Who is this Woman?[edit source]
First, I have NO "dog in this fight," so if everyone agrees she is Ai's actual grandmother, I will not engage in Edit Wars or otherwise demonstrate symptoms of butt-hurt and dyspareunia. ^^, Heck, I would love it if someone cited, say, an interview with the creators that state, "yeah, she's her grandmother, we didn't expand on it, baka!"
1. As I note in my edits, we never see her in the flashbacks. So if she is her actual grandmother, she died before Ai reached the age of seven. Perhaps she was on vacation? ^^,
2. I do not add this since it would perhaps be too much, but she does not seem to react at all to Ai's remembering of her past. To which, some of you may cite her "deal" with Hajime which we never get the details. Fine! I see your deal and raise you the fact though she shows apparent concern for Ai, she never delves into specifics. For example, she never tells Ai she appreciates what she does for the soul of her parents--one of them being her child!
3. Why would humans who see her be horrified?
4. Pro-Ai's Grandmother People may retort that it is "odd" that she is not around when Ai is "gone." The "New"--for like an episode ^^,--does not interact with her. That does suggest a connection between her an Ai more than "Old Hag" spinning her wheel of fate.
All that blathered and pontificated, it may simply be the creators had no interest in explaining it which, in a way, is more interesting. Just like we do not know where Hajime "went," what happened in the years when Ai was "gone," or exactly who the [CENSORED--Ed.] Kikuri is aside from a vessel for the Master of Hell.
Have at it!