I Care For A 3 And A Half Year Old

I care for a 3 and a half year old twin sister who has extreme tantrums and lately seems to switch off and on from destructive to sweet in an alarming way. I also notice lately that her voice changes and also her facial expression from engaged and angry to trance like and vacant. Her behavior greatly concerns me as I have taken care of many children her age and have never experienced such erratic, perplexing behavior.