Amaenbo Shimai no Omutsu Heya

The protagonist, looking for a job, comes across a nondescript-looking apartment, with a note on the door saying "Please take care of the girls."

When he comes in, he finds two cute, childish girls in need of an "Onii-chan" to tend to their various needs.

And thus, he begins a new life as a full-time babysitter for the energetic Chika and shy Misato.