Mayfirst...I am so glad I read your post. My Kobo has a similar problem with the screen on the top right hand side of it and it has the EXACT FCC ID number as your sister's. I have always looked after it. I have a protective case for it and always keep it in my purse which is never dropped.
I am going to call Kobo and ask them about it because I cannot think of how the screen was broken if it was never dropped.