Isabella's letter to Catherine is so artificial that it reads slightly out-of-character for a person who is usually on the lookout for herself.

It is obvious that Isabella fell "head-over-ears" in love with Captain Tilney, and was still in love with him at the time she wrote to Catherine. She mentions the "Tilney" name three times in the letter while she only refers to poor James as Catherine's brother. She is so obviously over James.
I wear nothing but purple now: I know I look hideous in it, but no matter — it is your dear brother’s favourite colour. Ch. 27.
Does Isabella really think insulting her brother's taste is going to convince Catherine that Isabella is still in love with him? If she is really grabbing at straws here, Isabella seems to be making a very feeble attempt- it is not at all up to her usual standard.