Marianne does, constantly, behave the way she believes a person with great sensibility ought to act. This is not to say that she has not come to the point where she has made herself believe these are her real feelings and her genuine emotions, but to have 'rules' for her behaviour--just as the part you quoted--shows exactly that.

She really was upset enough to cry all night, but she also believed that that was how she ought to act under the circumstances, because that type of behaviour would be suitable for a person of great feelings. It would be inexcusable not to be have this way. Where is this notion coming from if not from preconceived notions of a prescribed code of behaviour?