She's more like a chum than a step-mother.

They all love her and Una adores her.
As for that little Bruce, Una just makes a perfect slave of herself to him.

Of course, he is a darling.

But did you ever see any child look as much like an aunt as he looks like his Aunt Ellen?
He's just as dark and just as emphatic.

I can't see a feature of Rosemary in him.