Regardless of how much the scanlator rewrote this for their own amusement, even in the original that must have still been a demonstration of the extreme level of politeness and detachment those two display to make sure their new family remains only so on the paper and they won't start to feel any particular emotional connection to their step-sibling or step-parent. It's kind of like they treat their situation like something forced on them from the outside, which, I guess, is partially true since it was their single parents who fell in love and moved together, so they want to act more like cordial coworkers than siblings. Not to mention Saki wants to move out as soon as possible.