Yoda Is Your Father said:
My theory is that Ben Solo had a younger sister and when he went bad they hid Rey to stop it from happening to her too.

I’m leaning more towards cousin because of the look Luke gave her and I think she definitely was at the Jedi Academy and his minimalist training. Maybe a rudimentary practice in Shii-cho which to my recollection is the form that Jedi’s learn first in terms of lightsaber dueling. After Kylo Ren and The Knights of Ren destroyed it all her mind was wiped by Luke. I also think the part in the Force Flashback is a false memory I think she was 10 - 13 when her mind was wiped. This is the wildest of speculation.