Looking for a Book (Traveling Through the Military Marriage Years with an Arch-Enemy), the Male Protagonist Is Very Doting on His Adult Brother

Content: It started with a friend who was a real-life enemy. After traveling through the book, he became the heroine's brother. The heroine dressed up as a newlywed and got married in the army on the night of the wedding. The hero was single and had a younger sister. The hero's mother kept bullying the heroine and asked her to work. Finally, she persuaded the army and her rival to come to the island and found out that it was a game.