Looking for a Korean webtoon

Hi all,

SPOILER WARNING

There's a webtoon that I've been looking for but I can't seem to remember the title.

Details: there's a girl with blonde hair about shoulder length who is neighbors with a guy and his sister, both with black hair.

Details but more spoiler-y: >! I think the little sister's name was Mina or Min-Ah but I'm not too sure on that detail. They all go to the same high school and the main character (blonde girl) isn't the greatest student and needs to join a club so she and her neighbor and little sister join some sort of weird club where the president has pink hair and glasses and is very smart and top of her class but the club is really strange, and they have water gun fights at her house during summer vacation during one of the arcs I think. !<