The sister was rude, and is acting like a 15 years old, where she doesn't want her brother to get close to her but also doesn't want him going away but she isn't wrong calling out Noa.
Noa has been clingy and is doing everything a Menhera would do, I would have to read the manga again but I'm sure she already asked Rihito to date her once, and he shut her down, like 2 chapters ago she asked him to share an apartament together because she can't handle the being alone 3 days, the whole "first name" thing, we know how much she texts Rihito, and lets not forget we learnt like, 6 chapters ago that the entire manga up to that point happened in 2 months, how many times did she take him out to drink after work, Noa is doing everything to do she did to her boyfriends to Rihito apart from physical things since we know she was clingy and the only thing she refused was sex because she was scared, but she still is the type of girl who wants cuddles and hugs, the ear pearcing episode already had Rihito call her out on how she likes to get close and personal, the only reason they aren't dating is because of Rihito's, the moment he's okay dating her they WILL date, she completely broke that door and left the way open, the worst thing the sister does is complain about how can Noa crying because "How can a 20 years old make a 27 years old cry" and her atitude when Noa asks her whats her favorite K-pop group and she instantly goes into gatekeeper mode, its not her place to decide who can or can't date her brother, but she is right calling Noa on her bullshit of "not trying to date" him