As Julia Catherine pointed, Isabella projects herself onto others:

you mischievous creature, do you want to attract everybody?

and JA says that Eleanor "seemed capable of being ... at a ball without wanting to fix the attention of every man near her", which is intended to make a contrast with Isabella.

Perhaps she's trying to make James jealous? This conversation is made in his presence:

“Now, Mr. Morland,” for he was close to her on the other side, “I shall not speak another word to you all the rest of the evening; so I charge you not to expect it. My sweetest Catherine, how have you been this long age?

And also at the ball, she's with James when she tells Catherine to point her Mr. Tilney: