A not-related-by-blood menhera sister
They are in fact blood related, this was cleared up when people raised the question a few chapters ago. She's his mom's sister's daughter. They are cousins but have known each other basically their entire lives because she used to hang out at their house when they were younger so he calls her big sis (also his family basically adopted her at this point).
Tanuki rose a valid concern, my wild take is she is a real psycho, an actual grown adult posing as a highschool girl for her deviant purposes
She's in the school system and was suspended so unless she somehow faked her entry forms
just to fuck with Tanuki whom she only really started interacting with when he took it upon himself to go to the sports festival committee which she wouldn't have had any clue about (also Tanuki mentions seeing her in class but never paying her any mind) then this is some weird cope.
I really don't get why people think there is some twist that's going to happen with her when her purpose in the story is abundantly clear. She's Tanuki with tits, she's his reflection. She exists to highlight
to Tanuki how fucked his own obsession with his sister is by having somehow else do it to him. He apparently lacks the self awareness to realize this though.
On a lighter note, the instant she pulled out 4 tickets I knew she was going to invite the best boy to go as well. If she's as meticulous and calculating as Tanuki is she would have done her research. Friend-kun is the best character and it isn't even close. Mochizuki is a close second mainly because she's also eye candy. Komaru needs to start developing some independence and confidence otherwise she's just going to become dependent on Tanuki which isn't necessarily a better place than she was previously. Hopefully becoming friends with Mochizuki gives her that push since she's good at poking weak points and riling people up and if nothing else Komaru needs something to get her to stand up for herself.