This sentence doesn’t mean that Mrs. Bennet only visited Jane: it means that when she did visit Jane she talked with “delighted pride” of Elizabeth’s position at Pemberley, and the reader should be able to guess with what effect.
You’ll recall that the Bingleys “remained at Netherfield a twelvemonth” after their marriage, and Netherfield was three miles from Longbourn; whereas the Darcys repaired to Pemberley in Derbyshire, which was two or three days’ journey. Given these distances, we can imagine that Mrs. Bennet visited the Darcys once or twice a year, but the Bingleys once or twice a week. Whatever her other qualities, Mrs. Bennet was not outstanding for her tact and discretion, so that the reader should be able to guess that the “delighted pride” with which she talked of Elizabeth’s success in marrying a man with £10,000 a year might have come eventually to impose even upon the good nature of Jane and Mr. Bingley, who had but half that income.
The narrator goes on to say that Mrs. Bennet’s frequent visits were eventually too much for the Bingleys, and after a year they moved to a safer distance from Longbourn:
Mr. Bingley and Jane remained at Netherfield only a twelvemonth. So near a vicinity to her mother and Meryton relations was not desirable even to his easy temper, or her affectionate heart. The darling wish of his sisters was then gratified; he bought an estate in a neighbouring county to Derbyshire, and Jane and Elizabeth, in addition to every other source of happiness, were within thirty miles of each other.