Pablo Hidalgo answered this on Twitter; according to him, she represents the Alderaan sector, rather than the planet:
@BartlamTyler: I've nearly finished reading bloodline a I still don't know what planet or sector Leia represents if any
@pablohidalgo: Same as when she was in the Imperial senate: the Alderaan sector.
It seems like a misconception1 that Senators exclusively represent planets; on Twitter, Pablo Hidalgo notes that senators can represent either planets or sectors, based on such factors as population, among others:
@thesupremepablo Do senators represent sectors or systems? Trandosha and Kashyyyk are in the same sector and each have a senator.
@pablohidalgo Depends on population and other factors. Amidala and Palpatine before her represented the Chommell sector.
This, of course, leads to the question2 of precisely who she was representing; there are several (not mutually exclusive) possibilities:
There were other inhabited worlds in the sector, and Leia represents them3. Although none are specifically identified in the new Canon, we know it isn't impossible; both Eriadu and Seswenna - respectively the homeworlds of Grand Moff Tarkin and Admiral Motti - are to be found in the Greater Seswenna sector
Her position is symbolic, granted in recognition of her service in the Rebellion and her role in toppling the Empire
She governs the Alderaanian people in exile. We know that more than a few Alderaanian citizens survived the planet's destruction; one of them is an Imperial officer in the canon novel Lost Stars, and we meet several in the Marvel Princess Leia comic series.
This theory is given extra credence by the canon novel Life Debt, which features a chapter set in the "Alderaan Flotilla", a coalition of ships parked in the planet's former location and housing members of the Alderaan diaspora4:
Out there waits an infinite black space horizon of Alderaan, now reduced to rock.
The flotilla is safe, at least. Seven ships belong to the flotilla, now, including the Alderaanian frigate Sunspire. Another gift from the nascent Republic. Or, rather, another gift from their princess.
The ships float near to one another, gathering in a circle and protected by the deflector shield to keep out both the asteroids and, hopefully, marauders.
"This is our home. This patch of sky is ours. Our world was here. And the diaspora has returned us to this place.
Aftermath: Life Debt Interlude: "The Alderaan Flotilla"
This interlude ends with the Alderaanian government (such as it is) deciding to use the remains of the Death Star to build a more permanent space station home; though whether or not they did remains unclear.
1 Or a retcon, more likely
2 Posed by DaaaahWhoosh in comments
3 And I'm sure they're just pleased as punch that their only government representative isn't native to their world
4 Thanks to BBlake for pointing this out in comments