I don't dislike Akira but she indeed has her odd habits in how she deals with people. Although I love that she got to share the lolita fashion with the sweet Yuuki.
In my childhood, I remember that at my grandparents' house, my cousins and I used to play outside and one of our jokes was telling someone that we were going to throw them into the small ditch a few meters from the house. It wasn't common to actually push someone in, but countless times we had to find a way to retrieve a ball that had fallen in. A river, at least in my area, is much more dangerous and deeper, so even though I chuckled a little because it reminded me of my ditch pranks, the adults always warned us not to go near rivers, much less go in. So, if we looked at that scenario from a more realistic perspective, it would be a really a prank that could go too far.
Regarding the final pages, it's believable to think that Yuuki may have changed; however, I believe that she matured (mentally) a little earlier than her peers thus Akira retained a more childish perspective on her cousin.