Sorry for the late post.
This explains it a bit. As for me I never really heard that, but I do know there are many dimensions, is it possible for us to enter them? Not really sure. Scientists could be working on something, however, time hoping isn't true, I personally think some people wish it was. I could be wrong ; D if they do make a time machine/space-time travel that would be pretty slick.
If the banishing/protection spells didn't work, then maybe its the house..? Has she ever gone to a psychiatrist or ask a priest from a church? But if she doesnt feel as if the figure is evil he could be trying to make her smile, as experience with shadow figures I noticed if I'm crying figures will be different some will watch me, some will look sad as if they felt my pain, I even had one try to comfort me (literally felt his arms around me and him whispering "You can pull though"), and some will smile. I believe most shadow figures are harmless, they can't really hurt you but they can scare the wits out of you if they pop up out of no where.
Another could be maybe it's a past friend of hers searching for a way into the after life, but thats a religious point of view. Where when a person dies hes stuck in limbo and has to do a good deed to get into heaven. (If you have read Dant?'s Inferno, then you know about the 7 rings)
I googled 4 dimensions, as in a math point of view that's what I do believe from my understanding is what people are telling you so far we kmow three, height, width,depth, and finally the fourth dimension time it says in the link I sent you we can?observe and even detect it, unfortunately we can't control how to move through it.
Going back to a Astral projection aspect, if you astral project you can change your astral self but not your human self, maybe if your sister astral projects, she can get her answers about the figure? I know when I do I can ask them questiond and they'll respond.
I really hope this helps, if you have anymore questions send me a mail c: