She protected against the sister drinking session. The letters weren't switched then. It was switched before, when she helped that old lady at the entrance. The lady was bait that unfortunately our MC fell for.
She was expecting the sister to come. House Li knew she would be expecting her sister to pull something so they sent her to keep her suspicions low. The General took the drink so that she could let the sister steal the fake letters, not knowing that the old lady on the road had already stolen them.
Why is that so many Asian dramas/novels always involve hateful jealous bitches with plot armor???
She knows her lil sis is evil and she GRABBED A DRINK WITH HED WTF