You are Feodor's younger sister

One time he went to a meeting with Nikolai and Sigma about something you did not know. You saw both Sigma and Nikolai when you and your brother arrived at the meeting site, but this was the first time you saw each other. Nikolai and Sigma were surprised.

Nikolai: “She looks very similar to you, Fyodor~!” In a cheerful tone

Sigma: “Fyodor, did you even have a younger sister?”