Bellatrix should have noticed that the fire was lit where before it had not been. Little things like that, small signs of someone else being in her home, would normally have led to her searching the house, wand in hand, to suss out the intruder. However, this time, after the excitement of preparing for the battle, the maddening experience of watching Carina, and the elation that came with hearing of the success of their massacre, Bellatrix made one small mistake. She didn't immediately notice the signs of her intruder.
She was halfway in the house and had taken off her cloak when noticed the familiar blonde woman sitting in the chair facing the door. Instinctively her wand darted to point at the intruder, as she would to any surprise, but it was just her sister after all so it lowered a moment later. The fire's flickering light had allowed Bellatrix to overlook her for an instant, but now Narcissa was here, and now, she knew, the real trouble would begin. A true intruder would likely be less trouble than this.
"For Merlin's sake, Cissy, you nearly got yourself killed, you should give us a bit more warning before coming to visit." Part of her knew why her sister was here, that she was about to be the recipient of her sister's hellcat anger, but part of her knew that no matter how bright her sister's anger, it would pass. They were the Black sisters, after all, the only ones left, and they had to stick together. Traitorous cousins aside, they were all they had left.