What is the religious ruling on mistreating a maidservant, reprimanding her for the most trivial reasons, and cursing her, given that she is a Muslim, innocent, and does not understand the Arabic language?

Ill-treatment of servants contradicts the good conduct enjoined by the Sharia. Your sister must repent to Allah if she is mistreating the maid. She should be advised and her error gently explained. If the maid does not perform her work well, then either one should be patient with her and assist her, or she should be dismissed instead of being mistreated.