
What really happened to the brother ? Did he commit suicide or just run away ??

It remains a mystery, but I think he just left. A body would most likely have been found.
The fact that the park wasn't a forest was probably a clue. He couldn't have gotten lost in a forest. We are left like the sister--not knowing, wondering, hoping he's alive somewhere, maybe will get in touch in the future.
But really, he could be alive or dead, and we'll never findout.
http://www.mangago.zone/read-manga/kimi_no_mimi_ni_kokuhaku/an/kimi-no-mimi-ni-kokuhaku-chapter-1.html/5/
How is this not ?!?! Σ(  ̄□ ̄||)