Seeking Advice to Limit Internet Speed on Sister's PC Due to Neglect

The author is seeking advice on how to limit the internet speed on their adult sister's computer, as she is neglecting her severely autistic and intellectually disabled child to play Call of Duty (COD). The author has attempted to set up this limitation through their Internet Service Provider's (ISP) administration page without success. The situation has reached a critical point as the child has started breaking things in the house while the sister continues to play. The author, who has administrative access to the sister's computer, is looking for a technical solution to limit her internet access.