I believe that Frederick was unjust to Anne, she was only 19 at that time -- chap 4 --

"Young and gentle as she was, it might yet have been possible to withstand her father's ill-will, though unsoftened by one kind word or look on the part of her sister; but Lady Russell ... "

IMO, he was also unjust to Lady Russell in not distinguishing her valid concerns from that of Sir Walter's. Sir W simply thought it "a very degrading alliance", but Lady R was mainly concerned that its imprudence might result in "a state of most wearing, anxious, youth-killing dependance!" (especially considering his own earlier extravagance). Perhaps if FW and Lady R had liked and understood each other better, some acceptable compromise could have been reached.

In chap 10, he said to Louisa --
"It is the worst evil of too yielding and indecisive a character, that no influence over it can be depended on. You are never sure of a good impression being durable; everybody may sway it. Let those who would be happy be firm."

So Louisa got the idea that being firm means being obstinate even in silly things, insisted on jumping down the steps at Lyme -- chap 12 -- "I am determined I will", and hurt herself seriously.