Direct Translation:
* "If you don't treat me right, I'll call my grandma."
More Natural English:
* "You better treat me right, or I'll tell my grandma."
* "If you're not nice to me, I'll complain to my grandmother."
* "Don't mess with me, or I'll get my grandmother involved."
The English translation is a bit stronger than the original Spanish because "llamar a mi abuela" is often used in a playful way. In English, it's more common to use stronger phrasing, like "tell" or "complain," to convey the same sense of warning.