Does your GD actually fall asleep, and then wake? Or does the screaming start before she has gone to sleep.
I think there is a difference in this two things.
Also, it isn't that long since Christmas- so it seems there has been intervention and ideas swimming around in what is only a few weeks.
It may sound daft but is the room warm enough? I know my son had terrible nightmares in the cold weather.
She is the same at school, so this isn't a sleep related problem, unless this behaviour has a knock on effect because she must be tired, and when sleep quality is poor the mind can create hallucinations due to sleep deprivation.
The fact that the meltdowns are occuring for no reason leads me to think this is learned behaviour- she scream and people are thrown into a frenzy, it seems. So she gets the notion this is a good way to get attention, this in itself can be quite frightening to the child so the cycle continues.
I think this is far too early for therapy- has the therapy started yet?
The screaming starts when she goes to bed? I would suggest keeping her up a little later- engage her in relaxing but high attention activities- story books, no screens, no sugary things before bed etc.
At some point she will get sleepy- take her to bed and just stay in the room without paying her any attention.
Be calm, read a book, tell her you'll stay until she's asleep, and if she screams. tough though it is, ignore it and carry on doing a different activity in the room.
Eventually she will stop screaming and fall asleep.
Don't focus on the sibling either, don't blame the child for keeping everyone awake. This feeds the attention seeking behaviour. Reward the good behaviour, so she learns their will be a reward if she stops screaming.
If the beahaviour persists into a few months I would seek further help, but I think everyone is reading far too much into this at this stage.