Mrs Gardiner is one of my favorite characters in P&P. Though her part is small, it is important. She is very different from her sister-in-law (from both sisters-in-law, I should say!), and both Elizabeth & Jane obviously confide in her (through their letters).

In Chapter 26 Mrs Gardiner warns Elizabeth against being too quick to fall in love with Wickham. She knows that there is danger in giving this kind of advice, but she tells Elizabeth

"You are too sensible a girl, Lizzy, to fall in love merely because you are warned against it; and therefore I am not afraid of speaking openly. ... you must not let your fancy run away with you. You have sense, and we all expect you to use it. Your father would depend on your resolution and good conduct, I am sure. You must not disappoint your father."

Mrs Gardiner is appealing both to Elizabeth's sense and her desire to please (not disappoint) her father. These are important to Elizabeth, and the very sensible Mrs Gardiner knows exactly how to frame her argument.

After they have discussed the matter, the narrator tells us

they parted; -- a wonderful instance of advice being given on such a point without being resented.