Certainly the departure of Willoughby meant more than just a heartbreaking blow to Elinor, whose sister Marianne was deeply in love with him.
It also meant a huge blow to the rest of her family, since they had fallen from grace after the death of Mr. Dashwood. To the family, Willoughby was a welcome addition because he (expecting an inheritance) could have saved the family from the misery in which they were living. It also meant that Marianne would have acquired a rank in society, would have helped out her family and even Elinor and Margaret get rich husbands.
However, Elinor saw Willoughby also as her sister's true love. She was experiencing this herself with Edward Ferrars, and she was also tasting the flavor of infatuation the way Elinor was. These feelings of love bonded the sisters during these times, yet, seeing how Marianne became a broken woman after the Willoughby's leaving made Elinor just as sad, seeing how her sister became a completely different person.