Given the size of 19th century families, it is quite likely that there are a number of Lucas siblings, beyond Charlotte and Maria. Since Maria appears to be a teenager and we're told that Charlotte is 27, it's likely that Sir William and Lady Lucas had a few children over the decade in between.
We know there is a young son who comes to Longbourn with his sisters after the Meryton Assembly to discuss the night’s event. On that occasion, he says that if he were as rich as Mr. Darcy, he “should not care how proud I was. I would keep a pack of foxhounds, and drink a bottle of wine a day.” This statement implies that he's rather young, and we never learn if he has any elder brothers.
