SOLVED. YA Fantasy about twin sisters, Victorian-ish setting(?), one is the key and one is the 'door', accidentally swapped roles as c-section made younger one born first. [s]

[G]

Book one in a series - it's about two sisters, from a line of twin sisters who each get assigned the role of either the key or the door holding back evil of some kind. The one born first is always raised with like the influence of evil spirits, but because of these two being born via c-section the one who's meant to be younger was technically born first so got the role that's usually given to the evil twin (or vice versa), but she doesn't have the influence of the evil spirits so doesn't turn evil.
It's a historical setting of some sort, I think Victorian. They live in like a manor, I believe their mother is dead and their father isn't really around much. The main sister (non-evil one) has a love interest I believe. She doesn't really know throughout the book that her sister is evil, but they have kind of a verbal confrontation at the end and the evil sister chooses to leave I think.

Read around 2010, don't think it's much older than that. I thought the title had something to do with Key in it but I'm not certain.

[Becca]

Pretty sure that's Prophecy of the Sisters by Michelle Zink

[G]

Becca wrote: "Pretty sure that's Prophecy of the Sisters by Michelle Zink"

Yes, this is it! Thank you!!