Our protagonist, Ren, and his younger sister, Rio, essentially live together.
Lately, Rio has been acting strangely aloof, but they've been living a relatively peaceful life.

Then, one day on a holiday—
Rio, in a panic, wakes him up (as usual), rudely.
He rushes to the living room and finds his sister's best friend, Yuuri, there.
When he asked her what was going on, she said,

"As of today, I'm your little sister,"

And so, without really understanding what was going on, they ended up living together.

For some reason, the number of little sisters suddenly increased.
And the real little sister became even more grumpy.
But as Yuuri clinged to Ren, Rio's attitude gradually changed.

"...It's all your fault, big brother."

Then, what should have been peaceful days suddenly become exciting...
And so, their lives turn sweet and erotic.

[From Getchu]