How is the fissure in the house symbolic?

The fissure in the house is symbolic of the issues within the Usher family. The house, at the foundation level, is unstable, just like family. Poe even states early in the story that the villagers use the term "the House of Usher" to refer to the family and the house itself. There were early allusions to incest within the family as well, leading to the family being just as unstable as the house.