Who comes that unavoidably causes the number of people at the party emma three to increase?

Mr. Knightley's daughters must accompany him, which increases the number of people at Emma's party.

"The two eldest little Knightleys were engaged to pay their grandpapa and aunt a visit of some weeks in the spring, and their papa now proposed bringing them, and staying one whole day at Hartfield—which one day would be the very day of this party."