From DGT:
Because Yukino's mom is a housekeeper for the Renjyo familiy, she has to suffer the pain of hearing her mom praise the family she takes care of. And this is why Yukino has been trying to beat Renjyo Tamaki for most of her life, so her mom would care more about her than the Renjyo family. But when her mom gets sick, and she has to take care of the Renjyo family instead, her goals change a little.
