This just reminded me of something about Hanako.
As far as I'm concerned, no normal teenager would be this clingy towards an older sibling. Even if they are super-close, I'm pretty sure that they wouldn't try to sabotage their sibling's love life just so they can continue hanging out together. Most people would just talk out their worries with each other, reassure that they can continue to hang with each other, and boom, end of their problems.
The only explanation I can think of is if Hanako suffered from a mental illness that caused her to be so irrationally clingy to her brother (like
Dependent Personality Disorder). It could explain why she's away for such a long time- they sent her to a different school to try and get her to be more independent, but it backfired. Sadly this would actually make her somewhat sympathetic and have actual character outside of 'happy, anime girl', so there's a very high chance that Alex will just say that they're 'super, super close' instead.