The Wards on Grimmauld Place are ancient, blood-supremacist, and sentient. They violently reject anyone who isn’t a Black. When the Order first moves in, it’s a nightmare. Mad-Eye gets burned by the doorknob. Emmeline trips over rugs that move on purpose. Even James, pureblood as he is, gets hissed at by the shadows.
But the first time Remus walks in? The house is silent. The gas lamps flare to life to light his way. The dusty curtains part to let him see out.
Sirius is terrified at first, thinking it’s a trap. He asks Kreacher why the house hasn't attacked yet.
Kreacher mutters, looking at Remus with confused loathing, "The House protects the blood. The House protects the heir."
"He isn't the heir," Sirius argues. "I am."
"The House cannot tell the difference," Kreacher croaks. "His magic tastes exactly like yours. You have mixed your souls so thoroughly, Master Sirius, that the ancient magic thinks he is you."