Mr. Gardiner is Mrs. Bennet's brother, but they're not much alike:
Mr. Gardiner was a sensible, gentlemanlike man, greatly superior to his sister, as well by nature as education. The Netherfield ladies would have had difficulty in believing that a man who lived by trade, and within view of his own warehouses, could have been so well-bred and agreeable. Mrs. Gardiner, who was several years younger than Mrs. Bennet and Mrs. Phillips, was an amiable, intelligent, elegant woman, and a great favourite with all her Longbourn nieces. Between the two eldest and herself especially, there subsisted a particular regard. They had frequently been staying with her in town. (25.2)
Besides giving Elizabeth and Jane some much needed parental figures—seriously, those two need at least one sane and involved adult in their lives, right?—the Gardiners can also be seen as part of the whole discussion about what makes a gentleman. They're born into the middle class—not the gentry, like Darcy—and Uncle Gardiner makes his money by working as a lawyer rather than by inheriting it. Yet, when Darcy meets them, he is totally floored by how he would never in million years be able to tell that they aren't born gentlefolk.
Whew. That gives us a little hope for our own class status.