My eldest DS did this when he thought that his younger brother was getting more attention that him. He was almost 3 at the time.

He grew out of it. I did make sure my DH and I had more one-on-one time with him. We would also sit and just have conversations about things with him, like cars, toys, his new shirt etc. If it was really interesting, he would talk normally.

However, if you are really concerned then take her to the GP.