From Aerandria Scans:

Year 11, Showa Era—Hikari, the illegitimate daughter of the Hazumi household, received a marriage proposal. While she still has feelings for Kousuke, a distant relative, she couldn’t refuse the proposal because she knew how Suzuko, the legitimate daughter, was feeling. She receives support from her twin sister, Akari, who is being brough up by another family. They secretly continue sending letters to each other, but what happens…?