Homeschooling parents, how do you respond when someone questions your choice to home school?

I think if it wasn't hmeschooling, she would have the same attitude about another subject. Some people are just that competitive or perhaps she feels inadequate and is putting you down to make herself feel better. Try to figure out the root of what is the cause. Then you should probably accept that is just how she is or at lest figuring it out could help you figure out how to handle it.