I Met My Enemy at the Engagement Party

Two legendary sword clans are lifelong rivals.
A former detective is reborn as the treasured daughter of one clan, only to be betrothed to the heir of the other to force a truce.

At the engagement she meets someone tied to her past life, and the rival remembers that past as well.
Jealousy from powerful allies turns the political match into a perilous, slow-burn enemies-to-lovers game.

(Source: MangaUpdates, edited)