Oh boy -- such a big topic! First of all, how do you define "spoiled brat?" Child psychologist Ron Taffel once described it for me as a child with a chronic set of entitled expectations. I like that definition because it distinguishes between the typical child who has an occasional tantrum -- depending on the age, maybe once a day -- and the child who tantrums repeatedly, regularly, almost predictably, more than once a day.
OK, so I hate to say this but it is unlikely that any one person is fully responsible for your child's attitude of entitlement. More likely, sevseral adults in her life contribute in one way or another. But you know what? This is not about assigning blame. A better way to deal with this is to decide, hopefully together as the adults who love this child, that you are all going to make some changes.
Where are you each on the continuum of parenting styles? Making this determination can be a helpful guide if it enables you to each identify where you fall and what compromises need to be made. What's most important to keep in mind is this: children feel most safe and secure -- and are best able to thrive and reach their developmental potential -- when their caregivers set appropriate limits and are consistent in enforcing them. Children tend to act out and tantrum because they are unsure what the limits are and therefore feel compelled to push the limits as way to find out: When are the limits? What do I have to do to get them to set the limit? What if I do this? What about this? Will they set the limit this time? What about this time?"
In this article, I list what I think are the most helpful strategies for avoiding power struggles, setting limits and being consistent in your response.
For more ideas, check out Richard Bromfield's book, "Unspoil your child Fast."