How do you guys deal with spoiled children?

I have a cousin who is THREE years old and she gets on my nerves all the time. She gets everything she wants, it's either her way or no way. If there's something she likes and someone has it, she would act all cute and ask if she can see it, and when they give it to her, she would not give it back whatsoever. She would cry for god knows how long, and her mom would get mad at that person, but not her daughter. Her mother is such a hypocrite, telling my mom how I'm only 16, she shouldn't be buying me lipgloss, but when HER 3 year old daughter wants one, she doesn't hesitate getting her one. When her mother isn't around and she misbehaves, one of my other cousins or I would snap at her, and she would cry so loud, and an adult would come in, and she'd tell them that we hit her. Seriously, how should I deal w/ a spoiled brat like her?