This time, we have a love story between Kyouka Tachibana and Yuu-kun from "Tachibana-san's Circumstances with Men". Since Kyouka is the main character this time, the scenes featuring Yuika and Koharu are simplified.
[Character Introduction]

Kyouka Tachibana
Kyouka is the mother of Yuika and Koharu, estimated to be in her mid-40s but appearing much younger than her actual age. She raised her two daughters and Yuu alone after her husband passed away. Although she is an excellent company employee and speaks intelligently, she is weak to sexual pleasure. She has an attractive body that draws men to her, with a bust size equivalent to several meters (K-cup).

Yuika Tachibana
Yuika is currently a 20-year-old college student. She is Koharu's older sister and Kyouka's daughter. With her black, long hair, tall height for a woman, and slightly stubborn beauty, she is often called a "cool beauty." Her bust size is slightly smaller than Kyouka and Koharu's, but still over 90 centimeters (H cup), inheriting Kyouka's beauty and high sexual appeal. She tends to have a cold attitude towards men other than Yuu.

Koharu Tachibana
Koharu is Kyouka's daughter and Yuika's sister. She is 20 years old and a university student. Compared to Kyouka and Yuika, who have sharp beauty, Koharu has a gentle and laid-back appearance. She is academically excellent but has a slightly airheaded side to her. Among Kyouka, Yuika, and Koharu, Koharu is the most vulnerable to sexual pleasure. She is a 〇〇〇〇〇〇〇ed woman who selects men based on the size of their penis.

Yuu
Yuu is a 16-year-old boy who lives near the Tachibana family. He has had a relationship with the Tachibana family for a long time, but he started living with them after his grandfather, who was his guardian, was hospitalized. Both of his parents have already passed away. He had sexual relationships with Kyouka, then Yuika, and finally Koharu. Now, they have sex every day with the four of them. He is worried about his small and unimpressive appearance. His penis size is small (S size).

Fantia Sample
https://fantia.jp/posts/1952385