https://www.dlsite.com/maniax/work/=/product_id/RJ358259.html
https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_216306/
https://www.pixiv.net/en/users/3117035

Machine translated plot summary:
Takashi Nishiyama is a businessman who has left his hometown and lives alone in Tokyo. His first love is his older childhood friend Yuka Nishiyama, who is now his brother's wife. During a visit, Takashi is able to spy on Yuka's love life as she and his brother live their married life without problems while Takashi spends his days jerking off and thinking about what could've been.
One day, he suddenly receives news of his brother's death from his parents. Yuka is depressed and Takashi is heartbroken, but then he is told an even more shocking fact: His brother and Yuka were unable to have children. As the family's eldest son and his wife had not yet been blessed with a child, according to family law, she must be impregnated with the younger son's seed. Takashi is bewildered by the sudden order to impregnate his dead brother's wife, but Yuka, despite her embarrassment, agrees to adhere to the family's law.
They enter a relationship purely to follow that law but the widow ends up gradually falling for her brother-in-law, both physically and emotionally...

Originally released in November 2021.

002 - 195: Text, with glasses
196 - 389: Text, without glasses
390 - 583: No text, with glasses
584 - 777: No text, without glasses

PDF is included in the torrent.