The one where Jen's mother dies, and where Andrew dies.
The body count over the space of a few books almost resembles the last scene of Hamlet. Both Jen's parents, Mrs Shirley, old Lady Marchwood, Andrew Marchwood, both Rosamund's parents... have I forgotten anyone? EJO was evidently having a clearout of surplus characters.
Teasing about Ken might just about have been acceptable for someone who had known Jen continuously for 7 years, though it would still have been seen as a bit vulgar; but for someone who had known her for a few months 7 years earlier and has just turned up again, it was not good manners at all. Things were very different then...
Even nowadays, it's not very nice to be teased about a potential new boyfriend by people you hardly know. Della and Dick weren't bothered about Jen's feelings, they just wanted some juicy gossip.
The situation was complicated by the fact that Ken was by a long way the most eligible bachelor in the local community. If Della and Dick had continued to gossip and a garbled version had got back to the Marchwoods, it could have been hideously embarrassing for Jen.
I don't recall the dress incident; maybe I had an abridged version of the book in question. But there's a Dress Incident in Antonia Forest, when well-off Miranda gives Nicola an expensive dress, and Nicola isn't at all sure that her mother would approve of her accepting it. And Mrs Marlow is far more relaxed than Mary Dorothy.