I don’t think Henry believes his brother is blameless for his actions or is helpless in the face of Isabella’s charms. In Ch. 19 he said “My brother is a lively and perhaps sometimes a thoughtless young man” and in Ch. 25 “I hope he has not had any material share in bringing on Mr. Morland’s disappointment” which suggests a belief Frederick’s behavior contributed but is not the main culprit in James’ disappointment. Although Frederick did wrong to James it is Isabella who broke faith with him.

“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.” (Ch. 27)

In Ch. 27 Henry has very little to say (nothing good) of Frederick’s motives in pursuing Isabella describing it as a vanity—meaning, I think, he pursued her because she invited and he is not one to pass up an easy dalliance. He further equates Frederick’s vanities with Isabella’s which are unprincipled and confirms to Catherine his brother only courted Isabella for the sake of mischief which I think is censure in itself.

“But we must first suppose Isabella to have had a heart to lose — consequently to have been a very different creature; and, in that case, she would have met with very different treatment.” (Ch. 27)

When Henry says Isabella would have received different treatment from Frederick if she had been a different creature I think he is making a statement about the behavior of both. She probably flashed her eyes and then tried to attach Frederick with the same coquettish behavior she employed with James and inturn he felt licensed to treat her in the exact same manner. This is not suggesting Frederick was powerless to refuse Isabella but that he chose to behave badly with her because she was willing and inviting.

“And how strange an infatuation on Frederick’s side! A girl who, before his eyes, is violating an engagement voluntarily entered into with another man! Is not it inconceivable, Henry? Frederick too, who always wore his heart so proudly! Who found no woman good enough to be loved!” (Ch. 25)

Frederick’s attitude smells faintly like Henry Crawford’s in MP. The Miss Bertrams were willing fodder for his temporary amusement but not the “sort of woman it is that can attach me, that can attach a man of sense” meaning (my interpretation) no man whose sense is intact desires an artful, vain and unprincipled Maria or Isabella for a wife. So to paraphrase Henry: Had Isabella behaved like a Catherine or an Eleanor she would have garnered respectful rather than the trifling treatment she received from Frederick.

Further cross-novel discussion should mosey on over to Austenations. Thanks! (;D)