The Question and Answer section for The Fall of the House of Usher is a great resource to ask questions, find answers, and discuss the novel.
The houses are old and spacious.... mansion. They are both inhabited by adult siblings, and they are each lost to those who live in them.
Nature is reclaiming the rotting structure and its surroundings.
I don't understand what you mean by "taken over". Both the setting and the house suggest that, like Usher, the house is rotting back into the natural, rather dark, setting.