Why does it matter so much that Madeline and Roderick are twins?
It matters that Madeline and Roderick are twins because of several reasons:
The assumption that they may have been conceived within a consanguineal line
The assumption that they "share the same inevitable fate"
The possibility that they will meet the same demise
They are reflections of each other
They are co-dependent and their melancholy is mutual
They are extensions of each other