] There’s an interesting bit of insight into the family dynamics as a result of Catherine’s refusal to go on the drive. After protesting several times that John is not in Bath to drive his sisters, he winds up doing just that, deciding to take Maria on the drive because Anne has “thick ancles” and presumably it would shame him too much to be seen with her.

I have a feeling that Isabella told John something to the point of "I expect a proposal ~ we are going for a drive." John hasn't and doesn't want to take either sister, but the wish of securing James wins out.