Regarding the person with short bob who calls Rihito "oniichan". It's not uncommon for moms to call their elder children "oniichan", "oneechan". A friend of mine calls her elder daugher "oneechan". I was confused at first but judging by her maturity she's most probably his mom.
I'm assuming it might happen in other languages as well. I can say for sure that it is not uncommon (although not super common) for parents in Brazil to call one of their children "mano" (bro) "maninho" (little bro) and so on. (Portuguese)