] So, what is going on between Isabella and Frederick Tilney, eh? After swearing she would not dance, James Morland being out of town, and Capt. Tilney being informed of such, they do indeed dance, much to Catherine’s surprise.

IMHO, Isabella did not really meant what she said when she was 'determined not to dance', but Catherine, who did not know this, was surprised when Isabella did dance.

] And, did you all catch Isabella’s comment about there being more than one way for she and Catherine to be sisters?
] That third method being, I assume, if she and Catherine both marry Tilneys.