When Erik heard his brother at the door, sneezing and snorting, he knew his work contaminating Harry's room had been unnecessary--Harry was already well on his way to Harriethood. The androgynous person trying to sneak into the house had already developed discernible breasts, indicating that Harry had been infected around the same time as Erika.
Sneaking was in vain as Erik stood in front of his new sister. "Hiya, Harriet. Enjoy your last day at boy's school? You're going to love learning to iron at girl's school. I'll bet you'll be a natural at man-pleasing."
Harriet's brief attempt to assert herself as a (former) man collapsed instantly in the face of Erik's dominant presence. "Get your crap out of my room." said Erik. Harriet, wide eyed with terror, went to fulfill her new brother's demands. She thought of all the things she, when Harry, had done to Erika, and the infinite possibilities for the new Erik's revenge,
Good, Erik thought, let her stew.