The Southern Poverty Center has been included in the so called extremist groups but I doubt the number is 500 that they quote. There are way to many. If Southern Heritage is a shameful reminder of slavery and segregation, you should read, both current and past history of the Northern States, slavery started in Massachusetts not only for Africans but the Irish & Scots and in many Northern States the institution was still in place after the war ended, under the U.S. National Flag. The war was not fought over slavery, it became a political issue after the war started and Lincoln’s proclamation was not very well received by the slave holding States of the North even though they were not included in the proclamation. (You should google it and read it.)
Your Grandmother is correct, you should understand symbols before you use them, for your interruption may not be the same as others. The new movie “Amazing Grace” will give you a better understanding of the African slave trade but since this is a flag discussion board you might want to read more then this post as to what is discussed here. A visit to Confederateflag.org will help you on the Confederate flag questions.
History and a little Vexillology goes a long way in understanding symboles of the past and how they change over the years.
A dog use to be a animal now its a dude but is it still a dog?