Susan, is she pregnant by that point? What if you couch her resistance as fear/sadness that her brother won't get to part of her baby's life? Or that he won't be part of her life? If she's motivated more by strong emotional connections than simply losing a more upper-class lifestyle, that would be a more sympathetic motivation, I think.
Heh. Baby Arthur Horatio is four months old at that point. And it's not her brother she's worried about. It's a set of relatives that didn't even exist in the draft of Lucy you read--an aunt, uncle, and cousins who took her in when her father died after her brother was grown but she was just 13. They're a likeable bunch--anyone would regret giving them up. When she thinks of home she thinks of them first and her brother second. And at least one of the cousins is an important and highly sympathetic secondary character. He wouldn't be on the scene for this part of the story, but he'd keep the cousins from being mere abstractions.
Could someone guilt her about a specific relative she is particularly close to?
Definitely a possibility. One of the cousins could even be visiting.
Anyway, it's not so much that she'd be thinking, "I can't do this because it'd involve giving up my upper-class lifestyle." It's more like she was brought up with the attitude that it's maybe OK to boink the Lower Orders on occasion after you've given your husband an heir and a spare (and here Anna's Great-Aunt Flora would've sighed reminiscently over the handsome and lusty gamekeeper they had back in the 1760's), but that it wouldn't have even occurred to her as a possibility to marry more than a step or two out of her class in either direction. I'm even planning to have her fantasize earlier about being able to marry Jack, but with all those fantasies either involving him as a long-lost duke's heir or her not actually being an earl's niece and baronet's sister.
And then when it does occur to her that there's absolutely nothing to prevent her from marrying him if she wants to, since she's over 21 and can make her own choices, she'll have a few understandable regrets over giving up her family before she decides to be brave and propose to him.