Allusion is definitely one literary element Austen frequently uses in Sense and Sensibility. One allusion can be found in Chapter 5 in which Marianne is saying adieu to Norland. Her long, dramatic speech at the end of the chapter is very reminiscent of an ode, even though it lacks a rhyme scheme. In fact, it is very similar to William Cowper's poetry, who is mentioned in Chapter 4. Some of these lines are very similar to Cowper's lines in The Task: Book III--"The Garden." For instance, the line, "But who will remain to enjoy you?" is very similar to Cowper's line, "Or tasting long enjoy thee" and "Happy house" is very similar to "Domestic Happiness."
Another allusion found in Sense and Sensibility is to Shakespeare's Hamlet (Ch. 16). A parallel can be drawn between Ophelia killing herself in Hamlet for love and Marianne nearly dying from a fever induced by a broken heart.