What is the relationship between Sera Masumi and Haihara Ai? Why does Conan say Sera and Haibara looks surprised?

The analysis is as follows:

1. Sera Masumi only protected Haibara Ai. Specifically speaking, the relationship can only be linked to Akai Shuichi. Sera Masumi is Akai Shuichi’s sister. And Haibara Ai's sister Miyano Akemi is Akai Shuichi's girlfriend.

2. Sera's identity is very suspicious, so Conan said Sera, and Haibara looked surprised.

a. First of all, when she first appeared, she called out Conan's name and knew that Xiaolan was the best in Kanto karate. The first thing can be heard from the sidelines, but what about the name? Before she was called Conan, neither Xiaolan nor Yuanzi was called Conan.

b. There is a picture that is very clear. After her reasoning "failed", she covered one eye [Sera's strange behavior] and stared at Conan with the other eye. Obviously paying attention to Conan's performance. Maybe she heard Kudo Shinichi's name from others, did some research, and might know that Conan and Shinichi have a more complicated relationship than ordinary family members (that is, she suspects that Conan is Shinichi).

c. Moreover, her reasoning failure in this episode is most likely intentional. When experimenting with her modus operandi, she deliberately pushed her wheelchair parallel to where Conan was hiding. Combined with what she said before about her "good ears", she most likely had deduced Conan by hearing Conan's new voice. That is Kudo Shinichi.

1. The TV animation "Detective Conan" is adapted from the comic "Detective Conan" created by Gosho Aoyama and serialized in "Weekly Shōnen Sunday". The animation premiered on January 8, 1996.

2. When high school student detective Kudo Shinichi and his childhood sweetheart classmate Mao Lilan went to the amusement park to play together, they witnessed the suspicious transaction scene of the man in black. Kudo Shinichi, who only peeked at the transaction, ignored another accomplice approaching from behind. He was given poison by the man, and when he woke up, his body had shrunk! If those guys knew that Kudo Shinichi was still alive, not only would their lives be in danger, but they would also endanger those around him. On the advice of Dr. Agasa, he decided to conceal his identity. When Xiaolan asked about his name, he changed his name to Edogawa Conan. In order to collect information about those guys, he stayed in the house of Xiaolan, whose father was a detective. The mysterious black organization... all he knew was that they used the name of wine as their code name.

In order to expose the true face of those guys, the little detective Edogawa Conan's activities begin! Reference: (Baidu Encyclopedia: Detective Conan)