What made Madame Danzard so angry about Lea brushing Isabelle's hair?

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Just as the two maids have an insular, intimate dependence of each other, the mother and daughter seem totally enmeshed. When the mother sees one of the maids brushing her daughter's hair (and apparently giving her pleasure), she feels threatened and usurped.

I'm not 100% but as far as I know, back then, ladies had personal lady's maids to take care of things like dressing and grooming. So, a housemaid doing something as personal as hairbrushing may have been considered to be way outside of her duties. But, that's just a guess.