When Elizabeth returns from Hunsford, she has much on her mind, and a great need to talk about it with a trusted friend: "It was not without an effort, meanwhile, that she could wait even for Longbourn, before she told her sister of Mr. Darcy's proposals." She waits until they get back to Longbourn, partly because Jane's own situation is tangled up in the matter.
But there is one other person who seems to rate highly in discretion and good sense, and is not enmeshed in the situation: Mrs. Gardiner. The impression I got from their conversations in chapters 26 and 27 is that Mrs. Gardiner would be an ideal confidante. Yet Elizabeth makes no discernable effort to consult with her aunt. Does this seem odd to anyone else? I confess I never thought about this until this GR.
(The only explanation I have is that it would interfere with the plot -- if Mrs. Gardiner knew about Darcy's proposal, she might never have suggested visiting Pemberley, and then where would the story be?)