The world of Jane Austen's Sense and Sensibility is a complicated one, in which everyone fits neatly into minute little pigeonholes in the incredibly stratified, hierarchical class system. Imagine this system as an enormous card catalog, in which every single person we encounter fits somewhere – and moving them to another place is quite a big undertaking, that involves shuffling everyone else around slightly and getting them all used to it. Her characters are all painfully, obsessively aware of their individual positions in society, and all of their relationships are marked distinctly by their varying amounts of power within the social system.
Movement between social classes is possible (though difficult) in Austen's novel; more important than birth or status is the presence of ambition.
Sense and Sensibility largely avoids the question of birthright or nobility, and instead focuses on wealth as a means to social status, implying a shift in the English class system of Austen's time from nobility to economic measures of power.