These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Ballet Shoes
Fair for Its Day: When Sylvia can no longer afford to send the girls to a prestigious private school, it never occurs to anyone to send them to a regular school instead. Rather they are kept at home and taught by their unqualified guardian until the doctors volunteer to teach them. The unspoken implication in the novel is that of course genteel young girls like the Fossils couldn't be expected to attend school with coarse working-class children. This is made more explicit in the 2007 film where the boarders speak of awful accents the girls might pick up at a free school.
Jerkass Woobie: Winnifred is a bratty little thing but it's hard not to feel sorry for her when Petrova gets the part of Mustard Seed when Winnifred is too late for the audition (Petrova not even wanting the role) and she's only trying to earn money to help her family.
Strawman Has a Point: Posy does act incredibly selfish when Madame has a stroke and therefore can't teach her ballet anymore but her ballet career is another way of bringing money into the house and she was going to get those lessons for free so it's a huge blow.
Winifred reacts immaturely to Pauline getting the role of Alice over her, but then it would be upsetting and seem unfair to be told that you didn't get a role not because you did poorly, but because the other girl "looks right."
Being twelve and becoming aware that you're ugly is hard enough- that it's a valid reason for someone less qualified to get jobs ahead of you, when you're desperate for work, must be heartbreaking.
Values Dissonance: The book, published in 1937, uses a lot of language like "not really sisters" and "not your children" that many modern adoptive families would find heinous.
Possibly- in a further example- the constant underlining of their origins, and never referring to Sylvia as their mother- is because it's preferable to anyone thinking that Miss Brown's daughters are her own, and illegitimate..