Defiant behaviour ruining our family

Our 9 year old daughter is pre diagnosis for ASD. She is bright and can be incredibly caring but she is having regular moments of complete defiance. She becomes spiteful, cruel and ignores every request for her to follow even basic instructions. We try speaking calmly to her, explaining the reasons we need her to do as she is told, becoming firm and giving clear boundaries and consequences but she ignores it all and refuses to do as she is asked. This can go on for hours and it results in me and husband getting distressed with each other, neglecting our other child and it spirals out of control. I have PTSD and GAD and I find myself wanting to run away rather than face her. She says she doesn't like the person she becomes in those occasions but she can't get out. I sympathize but in those moments she shows no empathy or care for her actions and seems lost. I'm terrified this will tear us all apart. She seems so angry all the time and despite all our efforts to find her support and a way to vent it just comes back as this nasty defiant child who seems to hate us and will do the opposite of everything we ask. She is also starting puberty so that's added a new dimension to. I don't know what to do. Thank you for reading.