i predicted it was the mother somehow but apparently the brother is the most likely culprit. though the timeline's different so i guess we'll see what happens.
normal old man reporting in: normal people have a problem with big age gaps. she is being the weird one with her infatuation. i think a lot of manga/anime primes your brain to think a little less of these problems but this is a real-world issue and typically women are not like this at all.