I'm worried for my 18 month neice--VERY concerned!

Sounds to me as if you come from an extremely stringent fundamental Christian back ground and are reading something more into this than is needed.
In the beginning you may have had a case for a good Neurologist. Only time will tell now.
However as far as her waving, smiling and cooing into the window and into space, that is fairly common for a child during the time of self discovery.
It is also possible she does see the loved ones that have left their bodies before her such as a Grandmother or Aunt.
Children play off of the emotions of the adults around them more often than not.
You are looking for something and therefore are finding it in your niece rather than through your own experience.
It is kind of like when a group of preteen girls all get together for a sleep over and every shadow and wind has a scary affect.
What my suggestion to you would be , stop looking vicariously through the little girl for the answers that you personally seek.
And just don't give it anymore energy. I stress that again , Stop Giving It Any Energy.
Now I know you will not follow my advice, but you should for thee little girls sake other wise you will be putting something on her she does not deserve. Kids are natural.

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i just keep on making a whole bunch of this stuff and hand it out its amazing. its almost to good to be true. I know have no interferance in my meditation.
Free instructions very simple ingredients
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*R.I.P* the inventer
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more info about the first kind
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furthermore this orgonite that you may or may not make can also clean up electromagnetic pollution and any other kind of energentic pollution.

I had this with my daughter when she was crawling (about 8 months?)
She would crawl into our front room (which only had a treadmill in it)
and she would sit in this particular corner and 'talk' for minutes on end.
At first I thought she found something interesting in the room but she did it everytime I put her on the floor....shed crawl into the front room and sit in the corner and talk......
It was the same corner everytime she went in there...
And for a whie it stopped....and youd think she wouldve grown out of it...
NOPE....

Four years later..we moved into a new house, and she tells me things shes not suppose to know...like where one of my aunts is buried..she died before she got meet my daughter.
She also now speaks of my fathers aunty (name and everything..description..)
These two women died well and truly before she was born.

But it is frightening...with children involved..you dont know what to tell them......

I would get a test done for Epilepsy before you got overly concerned about all this. My friend has this. It's very strange when he has a seizure, but the eyes thing you described is pretty bang on. He says he sees things when he has them, but he has learned to ignore it when he forgets his pills... Also he ocassionaly screams when he comes out of a seizure, but he doesn't know why..