Usher has been having to hear his sister struggling and trying to escape being buried alive. Eventually Madeline actually breaks out and escapes her coffin, but we don't know how she actually did.
While Usher and the narrator were talking about Madeline, the door to the room busts open and Madeline is standing there. She rushes at Usher and attacks him. While being attacked by his no longer dead sister he has a heart attack do to him being scared to much, being scared to death.
After Usher dies from being attacked, Madeline dies shortly after Usher dies. The narrator rushes out of the house after all of that happened, just trying to get out of there as fast as he can. The narrator stops running and looks behind him. The house is in shambles and all broken apart. It seems like the house was connected to Usher and Madeline.