Hatsukoi Killer Kouryaku Keikaku

Sumi, a 5th grader right in the middle of puberty finds crashing at the house of their first crush, Kii nee-chan! They intended to show off their cool, grown-up side, but their plans are completely backfiring...!? Caught up and sometimes rescued by their overpowering big sister. Get ready for an overflow of love and home comedy as it all unfolds.

(Source: Kadokawa, translated)