"[Isabella] shocks Catherine by saying “You must not betray me, if you should ever meet with one of your acquaintance answering that description.” Isabella is referring to a young man with light eyes and sallow complexion as a possible husband. Why would Catherine “steal” such a man, when Isabella knew that she liked men with darker eyes, complexion, and hair?"

I understood Isabella's meaning differently in this paragraph. Firstly, I get the impression she is referring to James Morland (because she hints that it is someone of Catherine's acquaintance, and she has already talked of being partial to clergymen), whom Catherine could not possibly steal in that sense! I think the use of the word 'betray' in this sense is a betrayal of confidence. Isabella is saying that she does not want Catherine to 'give away' that Isabella fancies James.