Omiai ni Sugoi Komyushou ga Kita - Vol. 2 Ch. 19

[chojinra]

She’s pretending to have a communication disorder. She interacts normally in all other situations. That’s just her excuse to be able to date him without him knowing who she is.

Yeah, her public stone face is nothing but nerves, and not exactly normal.

As for how she is with family, well, it's family. I think she's grown to be herself around them.