] They were raised by the same mother, so from where does she get her good sense? We can guess from the father, but we don't really know.

It happens all the time that children are given unofficial labels (usually with the best intentions) by their family, and it can be very hard to break out of the assigned role. I can certainly see Elinor as a quieter, more thoughtful child than her volatile little sister, and soon being known as "the serious, responsible one, such a well-behaved dependable girl". While Marianne got her mother's attention by behaving just like her, Elinor would have been praised for her maturity.