< Catherine sees that Isabella's flirtatious behavior did not justify the Captain's.

Well put, and I agree. The Captain is a scoundrel.

In defense of Henry, both of them are grown men and since when did a younger brother ever influence over an older brother? I also don't think he completely exonerates Frederick:

“I have very little to say for Frederick’s motives, such as I believe them to have been. He has his vanities as well as Miss Thorpe, and the chief difference is, that, having a stronger head, they have not yet injured himself.”

However, I see now that Henry is a little too good at brushing unpleasantness under the rug in order to keep the peace:

"If the effect of his behaviour does not justify him with you, we had better not seek after the cause.”