That's the thing, to be honest. At first it started off as Yuki's unwarranted declaration of love towards her sister, but in the last volume, things took a drastic turn. I mean, I personally love the HikaYuki ship and I wanted the two to be together so badly ever since the two met, but it was... empty. Like the pairing was just thrown into the manga without a single explanation as to why. I thought that Miyuki's love for her sister knows no bounds, but it turned out that she somehow liked Hikari more and they somehow started dating and Yuri's somehow okay with it.
I love the two together, but I thought it would be more... bloody, since Yuri's so in love with her sister. And don't get me started on the epic battle with the aliens. I wanted to know what actually happened there.