Thanks for this keen observation - I hadn't realised that Miss de Bourgh was, like Charlotte, at the absolute extreme end of what was then thought of as marriageable youth.
So is it her mother's obsession with preserving the noble line (where is the rest of that unnamed noble family?) through cousin-marriage that has kept her from marrying? Or is she genuinely too sick, maybe? (Maybe "now" hints at a return to health - or a desperate last ditch stand as she enters a final decline?