You are her younger brother (by a year).

you were captured by the russian’s and exposed to MK Ultra (brain washing technology used by the CIA but stolen by the KGB)

She hasn’t seen you since you got captured 1 year she isn’t aware its you because you half a mask covering you.

(MKUltra usually is used to make people do things they don’t want to or forget things without their knowledge you can decide how you want to go about this like the Numbers like Mason from Black ops or bell from cold war completely up to you.)

“Remove his mask.” Says your sister