Sippy Cup is the third in a series of videos featuring Melanie's child-like alter-ego, Cry Baby. The first video of the series, Cry Baby, introduced the character and her family, and revealing (among other things) that Cry Baby's mother is an alcoholic. It is also revealed that Cry Baby's father is an adulterer, and as shown in the second video, Dollhouse, his cheating is the reason for the mom's drinking.
Cry Baby's mother turned heel in the aforementioned video, Sippy Cup, as she's shown drinking again before confronting and attacking her cheating husband and his mistress. The deranged villainess then tied up the illicit couple and later stabbed them to death with a butcher knife, leaving the bodies covered up in the kitchen. Cry Baby later walked into the kitchen and saw the bodies of her father and his mistress, and shortly afterwards, she became another victim of her mother's heel persona, as she was chloroformed and tied to her bed by the villainess. After this, the evil mother gave her a drugged drink from a sippy cup to clear her memory and make it appear that the murders were part of Cry Baby's nightmare.