What kind of dog is she? How long have you had both of them? Who did you have first?

Don’t use a baby voice. Baby voices only reinforce the bad behavior because they sound soothing and like everything’s ok. Everything’s not ok because you don’t want her to be scared and you do want her to be calm. A calm, firm, but gentle voice works much better along with praising her and treating her when the behavior I good. Can you put her in a sit or in a down and praise her, slowly letting Chester get closer? Or maybe you even need to star out with things that smell like Chester and put her in a down or sit and move the scented item closer to her.