You're correct, it does happen a lot. But this would require
multiple people to have amnesia.
Grandma, Amy, Liz, Kat & MC would all need it,
as well as his sister.
Even if she got it from the crash, we know that the other 5 aren't suffering with it. As clearly evidenced by them all recognising each other.
You can't just give it to one person as explanation for why she doesn't know them, then expect 5 others who aren't suffering with the same condition to also lose their memories of her.
Seems a bit of a stretch... Even without HP directly quoting the dev categorically stating that she's brown bread.
Even in a game where the MC uses a variety of wildly inappropriate tools to fix cars, logic still applies.