Is it normal that I cannot imagine a more perfect love story than two siblings?
Two people who know and love each other from day 1, cradle to grave. That is amazing.
According to statistics, when growing up, siblings spend more time with each other than with anyone else, including their parents. And since they come from the same family/household, siblings already have a lot in common by default: their national, ethnic and class origins, their history, upbringing, family tree, social circles, et cetera (and even medical predispositions, if they are blood related).
Perhaps, what also plays into it is the dramatism or sense of tragedy, because of our society's negative view of such relationships -- and thus of our two protagonists. They would give up on anything to be with each other. The world is against them, and they only have each other; each other is all that they need.