This.....isn't true. Just look at the whole thing with Isabel. She clearly shows her anger. She clearly shows her happiness once they sorted things out. She is just so sheltered and awkward that she doesn't know how to show the proper amount of both, and at what times to do so.
lol it was just overemphasis for the sake of effect. Of course she still displays some, but you could hardly call her expressive!
The whole point of her character is that she's the calculating one, to Luna's explosive one.
She's Michael to Luna's Sonny Corleone.
It was her love for her brother that drove her protective nature. But I take your point, she was a bit petulant/childish there.
We all know she will argue with her siblings, but spring to their protection if she thinks they are threatened. Same as Luna.
She now accepts Isabell as one of her siblings, rather than a threat, once it was presented in a logical way.
