Child acting out, what do I do?

My daughter is 6 and in the past month she has been acting out and it’s to the point where I dread starting the day. From the morning until late at night the behavior is extreme. She is defiant and aggressive. If she’s not getting her way or if her older sister looks at her the wrong way she flips out into a rage of screaming, hitting, punching, pinching, and spitting. She will hit to hurt (goes for the face, head, nose, back, and stomach). It’s really scary and upsetting. All because she is told no or is told to do something (like it’s time to take a shower) I have tried counting, using timers, being calm, saying I understand your upset, use when/then statements, but to no avail it doesn’t work and after the calm down, it starts again. This behavior started out of the blue, she would have frustrations and lash out, but nothing like this. A little back story, the only thing that significantly changed is her grandfather died. She does pull his picture out every night and holds it to her. But when I had a heart to heart with her after a meltdown she said that she is sad because of it, but she’s not angry because of it and she’s just really angry. Does anyone have any guidance? I’m looking into therapy, but after the holidays, is there anything I can help parent wise until then, or anyone who has had a similar experience with their child?