@Mikhail Also possibly, "Character basically at the mercy of someone using them as a toy" vs. "Character grudgingly humoring someone using them as a toy."
@Mikhail Well, in Oniishimai the situation was very literally forced on the big brother to try and make him be someone he hasn't been. Whereas in this case, Akira opened that pandora's box himself and his big sis is more speeding up the process he himself willingly initiated. Seems like semantics but it's a pretty big difference on direct examination. You can make a solid case the former is the result of essentially long-term brainwashing, while the latter is basically a good big sis nurturing a little sis she didn't know she had.
(I like both series but I totally get why someone wouldn't like Oniishimai)