Siblings don't know that second meaning of "The moon is beautiful" is literally "I love you"! Oh god, he just said it to all girls there... Oh boy.....
@Suisuigo Yes, it is a Japanese thing, you can google around for it.
The short version is that Natsume Soseki, a famous novelist, once translate "I love you" in English as "The moon is beautiful" in Japanese. He said that the direct translation couldn't convey the nuances of the phrase "I love you". It has kind of stuck around since then.