Usually because a maid is of a much lower status. To be more exact the viscount daughter is actually a "lady in waiting". Usually lower nobles will enter the palace to serve the princess or the queen, sometimes concubine and mistresses as well. Their duties mainly consists of assisting nobles higher than them. Though it's not often to see them "cleaning" as it is a servant's job.
A common maid, while excels at laundry and cleaning, often doesn't have the skills or intellect for finer craft such as tea brewing, conversation partners and mannerisms to stay beside a high ranking nobles, hence, a lower ranked noble capable of handling those task will be assigned. We can also understand a little bit on why she is so frustrated on having to serve someone of a lower ranking than her. As usually they are ingrained to serve those who are of higher rankings
Of course I hate the sister too haha, I also hate the viscount daughter who became a maid, can't they choose a maid in the palace who they can use for monitoring? Why use a noble daughter who clearly doesn't know the job.