Ohh - more modern than the Victorians? Is she familiar with either
The Chalet School series (Elinor Brent Dyer)
or
The Marlow series (Antonia Forest)
both perfect, escapist lockdown reading for a twelve year old.
Girls Gone By publish some of them, (the rest can be found via the usual monolith. Prices less terrifying than they used to be). They also publish books by Gwendoline Courtney including Sally's Family and Stepmother (which, with its original title Elizabeth of the Garret Theatre, was one of my all time favourite books as a child).
If she hasn't read them I envy her! If she has ...
The House in Norham Gardens (Penelope Lively)
Small Change for Stuart ( Lisa Evans)
I Capture The Castle (Dodie Smith) is absorbing, but it's only one book.
How about P. G. Wodehouse? Masses of gentle, lighthearted reading.
In truth most of the books I'd usually be recommending for this age are the opposite of gentle ...