From Impossibility:

Kiriko has always been overshadowed by her older sister Eiko. Eiko is cute, smart, and extremely popular unlike Kiriko. This manga is about Kiriko leaving her sister's shadow while blossoming in love! Although this sounds like a typical manga, the manga-ka enjoys switching to different manga styles to show extreme expressions and actions of the characters. Seishunchuu is a very cute and funny manga!

...Seishunchuu! Vol.01 Ch.001.1: Sisters(2) is an outlier of the rest of the manga. It is basically about the rivalry between 2 sisters. It has been passed down through generations that whenever a woman reaches the age of 18, they will grow moles on every conspicuous part of their body. WARNING: Contains horror and mystery.