You can explain the reasoning for behaviour without excusing it. She's nearly 14. Sorry - I don't agree with you. As for years of abuse, her father was in their lives as a regular presence until fairly recently. She didn't grow up being abused her entire life.
There are plenty of examples of kids the same age who are HIGHLY stressed who don't necessarily go along with what is being demanded of them. And kids from great homes/backgrounds who commit unspeakable crimes. I don't think being 14 is a pass.
But what would the consequences for Ella be had she not done what Alice demanded? Would Alice have sent her to bed with no food, taken her devices away, hit her? Threatened to send her to a care home? She's already been brainwashed by Alice to believe her dad didn't love her and has abandoned her. She only has her mother and her sister left, as Alice has kept them away from Ioan's family (and her side that she's in contact with aren't great).
We are shocked by Alice from just reading about her. Just imagine what she's like in real life. Now imagine living with her. Now imagine she's your mother, and her having power over you, and you're too young to get away.
I'm pretty sure Alice has been manipulating Ella all of her life too. And Ella and Elsie have witnessed their parents arguing all their lives, per Ioan's statement. That would have affected them badly mentally too.
What Ella did was wrong, without a doubt. No one is excusing it. But people on here using language such as saying she looks like a quarterback and has a smug grin doesn't sit well with me (I am not policing though, if that's your opinion you're entitled to it, all I am doing it pointing out why I think differently). She needs professional help, now, and to be removed from Alice's care.