Very good point by paperclip. You might also want to find a substitute for what your daughter is chewing on. She sounds anxious and needs something to soothe her. A small ball that she can squeeze would be really good. Gum is much better than metal. But essentially, the twin competition maybe getting to her. Find a way to help with that.
I think you must find out why she has no confidence. My son at that age ate paper. I found out later that he had a perceptual delay. We took him to an Occupational therapist who sorted out the problem. Today he is a very confident young man. The occupational therapist will take her through a number of tests to determine what is happening.
Best thing I did for my son.