Even my famous ex had a blended family. Three kids from three different wives and quite a few step mothers for each one of them
Four kids if you count the officially recognised bastard with the mistress!
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If you look at this from Ella's perspective, the moment Ioan left Alice would have said, "Daddy's left us for another woman! I don't know who she I but she has poisoned Daddy! He has abandoned us!"
Alice will have continued this narrative daily. When Ioan and Bianca came out with their relationship it would've confirmed Ella's worst fears that Ioan had escaped, had a happy new life and had abandoned her with a resentful, unstable and abusive mother. Ella probably hates him for that reason. She probably also blames Bianca thinking that Ioan abandoned the family for her and that Bianca is stopping their family from being together. Alice will be reinforcing this belief.
We of course know this is all bullshit. Ella doesn't. She is angry, resentful, frightened and living in an abusive home with (in her mind) no escape other than into the home do someone who doesn't want her and whose partner is the one who broke down the family and wants her gone.
I'm not saying this justifies her awful behaviour but I would say that's the reason for it. She needs to be taken away from Alice ASAP. A few weeks of living with Ioan and she might realise the lies.