I really think there's a tier for manga authors(possibly more-so japan but still in general just people) that when they write in something that society doesn't really care for IE: Incest, they tend to have a habit of giving flimsy reasons why it doesn't stay that way.
While I could see the HikaxYuki from a mile away but that was because I'm already savvy about authors doing what I've previously stated, they shoehorn(though in this case it wasn't SO much of a shoehorn) another interest in there and give a paper thin reason why their relationship blossoms more than the previous relationship that for better or worse seemed to be written as unbreakable. Such as--
--It was all an act.
--Just going along with it.
--Confusion
--More than best friends less than lovers(This one is really popular)
The reason I say 'Paper Thin' is honestly because the author waits until near the end of their series to give these reasons as they wrap up the main plot in what usually becomes a Unsatisfying 'Ship' Voyage.
In this case, if you notice, both sisters got their other lovers based on mirrored reasoning "Protection", it was a throwaway reason and though it might've been planned from the start the execution was slightly lacking from a standpoint of not being aware of the 'Incest but not really Incest' trope...I'm sure there's a Trope on TVTROPES somewhere that explains this.
This usually happens in Drama that involves homosexuality as a main point.
Like this one manga I read, can't remember the name, but essentially it goes like this.
--Boy Likes Girl(Almost obsessing over her)
--Boy gets turned into girl
--Boy still likes girl and continues to pursue girl
--Boy(as girl) gets male friend to help
--Boy(as girl) falls for male friend instead DESPITE GIRL FINALLY FALLING FOR BOY(as girl)
and it's written like we were supposed to expect it all long with a throw away reason such as "They were there for me through the whole ordeal" all to avoid a homosexual relationship(yes despite there being Yuri and/or Yaoi artist who's works are published in Yuri and/or Yaoi-centric magazines)
TL;DR
Authors notoriously give flimsy reasons for why an initial Incest/Homosexual relationship doesn't happen. It's likely a trope, look it up.