With all due respect, this comparison is absurd.
This story revolves around two people living together. The exact relationship between those two people is essential to the kind of feelings the game it's trying to arouse or the message it intends to convey. You can let your imagination run wild and have them be a primary school teacher and her now adult former student, but in a VN of this type that personal choice of yours will most probably lead to ridiculous situations/conversations.
When you read a book, you create your own mental image of the characters' looks, voices, etc. but I've yet to read a novel revolving around two characters where you never even know what kind of relationship they have.