Well, that route was kinda closed right away, but see?
This is a good setting for a poly ending, if we could be open minded (still way too few works with that approach).
It's not that they CAN'T be happy "normally". But the older sister, who through her worries stopped seeing her lil sis as a person. And the younger sister, who felt her older sister had lost her ability to smile.
Both of them broke free because of the same guy. In normal circumstances, that's already the open door for a romance...individually. But since there's 2 of them (forget they're sisters for a second), nope, can't happen.
Why not? They could be happy together.