Kunoichi Onee-san Shizune no Gohoushi Ninmu ~Sessha no Subete wa Waka-sama no Mono Suteki na Kodane de Oyotsugi o Sazukete Kudasai~

The protagonist, Toshiya Makabe, is your average, everyday student.
On the day of the school entrance ceremony, Toshiya is approached by his childhood friend and upperclassman, Shizune Kakei, who says she has something important to discuss.
That night, as Toshiya waits nervously, Shizune shows up—dressed in full kunoichi attire!
The Kakei family has served the Makabe family as a clan of shinobi for generations. Shizune explains that, to continue the family line, she needs Toshiya to help her conceive a child.
And just like that, Toshiya’s life of making babies with his kunoichi childhood friend begins!

[Translated from dlsite]