Personally, I'm not assuming that at all. I just find the idea appealing and I don't think the movie had to cover it. It would give her a conflict with whoever fucked with her memory, and could be a hazard that could turn her to the dark side a source of hatred.
Yeah I'm sure what happened with her family and why she was left alone will be part of the story somehow.
People make a few baseless assumptions about Rey:
1- Memory loss
2- That she was
dropped on Jakku
3- That she was once force-sensitive.
But the facts are:
1- No indication of this and is somewhat contradictory to her remembering her family left her
2- No indication she ever lived on another planet. In fact the movie makes it almost certain she never lived anywhere else than Jakku, especially if we recognize there is no indication she had amnesia.
3- No indication she ever was until the events of TFA. Strangely people don't assume Finn was force sensitive until around the same time, TBD if he is, but the movie tells us the force awakened suddenly.
So assuming things are as the movie indicates: she is NOT force sensitive until TFA, NOT amnesic, and was NOT dumped on Jakku, and assuming that Unkar would likely have given something in exchange for the kid considering that's what this character pretty much is (could be debt repayment, or a transaction, whatever), we can guess that:
1- Rey and her family lived on Jakku since Rey was born if not even earlier (there is at least ONE village of what seems to be normal people, where Tekka lived).
2- Her family left on a ship which they probably didn't own (otherwise they could afford bringing Rey aboard)
3- Rey is left behind to Unkar as part of a transaction, the transaction being over securing money to leave the planet but not enough to bring Rey as well, or Rey is left temporarily as a promise to Unkar that full repayment of a debt will be made on their return.
That to me is the most sensible take on Rey if we assume the movie isn't trying to fool us, which I don't think it is.
Rey's vision of herself being left on Jakku is about how the ship her family left in flies into what looks like fire/red-cloud, symbolic that something bad happened to them, and this is followed by Maz telling her her family is never coming back, which ties in to what she saw and why she freaks out.