"Senior, that's not proper; it's illegal,"
Dorothy flatly refused this alluring proposal from the senior with long black straight hair.
Although theoretically, almost any creature could serve as a witch's familiar and be driven by a witch, even if they are fellow witches, witches are after all a civilized race, and ostensibly, all witches are equal.
Naturally, using a fellow witch as a familiar was not explicitly forbidden by any written law, but it was an unspoken rule that all witches tacitly abided by. Even the Witch King would employ the services of the most ordinary little witch under the guise of employment, rather than issuing direct commands.
So, Dorothy certainly didn't have the courage to break this unspoken rule that even the Witch King had to follow. Otherwise, if it got out, she feared she wouldn't be able to mix in the Witch World ever again.