I think talented and ultimately successful kids being brattish was one of the unusual things about NS's books at the time. Upper-class kids were supposed to achieve massive success without being seen to try, but the talented NS heroines/heroes (although they are usually girls!) have proper drive and ambition.
The genteel poverty stuff might also have been something that wasn't really said.
Peaseblossom is what a later, less posh, more practical Garnie might have done.
What is the line.. something like "Their mother was one of those gentle, spoiling sorts, and they might have turned out loathsome if Peaseblossom, their mother's old school friend, hadn't come along and said "Come along old bean, I'll give you a hand"" (help me out here Footle!)
Translation: Bee either made a good marriage or had family money or both. Peaseblossom (real name what, Joan? Mollie?) had neither. Peaseblossom ends up as mother's help to posher school friend - thank goodness Peaseblossom's family did manage to pay for the education where she met a posh school friend, otherwise she wouldn't have had the connections to stay in her class (although perhaps she might have had more opportunities and more fun doing something else - senior nursing?)
Going out to work or doing other stuff that would be beyond the pale for your class would have meant - even if you decided you didn't mind for yourself - no more contact with any family and friends, no marriage opportunities. You'd have to build your entire network yourself - but the "lower" class people probably wouldn't want to be connected to you, unless they were also estranged from their class in some way. It would have been incredibly isolating.
(BTW I absolutely LOVE NS's explanations of how families have servants without ever using the "s" word for middle/upper class family helpers. Is it Apple Bough with the man and woman who are described as "a fairy, ask them for anything and they make it happen"?)