Wow, didn't know that each chapter had a different bewtle. Gotta wonder if there is any meaning behind it, or if the author just likes beetles
Edit: went down a weird internet hole about these beetles, and apparently breeding them is popular. Also her wearing the shirt may be a inside joke, about her yandere tendencies and the behaviours of said beetles. It seems the male beetles will aggressively hold onto something and refuse to put it down for a long amount of time. So maybe the beetle shirts are referencing her obsessing and refusing to let go of her brother. I could also be reading way to far into this and the author just likes beetles