Suki to Suki to de Sankaku Ren'ai

Meet Sousuke, our protagonist. He's just your average student, living in a rural town. Ever since his father remarried when he was young, he's been living together with his little sister Nanaru.
Together with his childhood friend Maho and their other friends, they have fun geeking out at school in the Otaku Club (or the "BC," as they like to call it).

Sousuke enjoys those tranquil, peaceful days... But then one day, only one month before summer break, that peace is shattered.

"I've missed you so, so much... Nii-san!!"

It's his blood-related little sister! His beloved little sister, who had gone with their mother after the divorce, and she's back! Suzu silently flaunted her status as the 'real' sister at school, at home, and out around town, while stepsister Nanaru was just left feeling awkward and out of place.

Stepsister Nanaru: "It's too late to come in here claiming to be the 'real' sister, I've already trained him, hohoho."

Real sister Suzu: "Fine! I'll just RE-TRAIN him!!"

"This is bad. This is VERY bad. My sisters are going completely haywire!"

But if that wasn't bad enough for our poor protagonist, now some upperclassman called Shiina, who 'forced herself' to fall in love with Sousuke at first sight, appears! And if you thought the childhood friend Maho was still normal, too bad! She's acting slightly bonkers too. What is our poor protagonist going to DO?!

This is only the beginning of a wacky, zany love triangle fiasco hot enough to cool the summer heat (no guarantees).