In the aftermath of the stolen chicken incident, I felt sorry for Sarah.
Sarah had to admit, she did not know how to write.
It seemed she could read a little, all be it she could not read in front of Rose.
In the first episode Rose hands her a list of chores and it seems Sarah can't read
https://www.youtube.com/watch?v=H2EH4eUTxLM&t=1200s
But the continuity could at times be inconsistent. Later we see Sarah trying to teach Ruby to read with some old ABC blocks. She also copies Hudson's signature to steal a bottle of wine (for Watkins), holding the pen quite well, in contrast to the earlier scene (after stealing the chicken) where she awkwardly holds a pen saying she can't write. Combine this with the fact that Sarah, as a character, is an exaggerator and isn't very truthful at times, at least in her first few episodes. This would sort of change when she returns to the household in episode 10.
I don't feel sorry for Sarah about the chicken incident! She's a brand new employee and she steals a chicken, not out of hunger but to sell!