For days, Narcissa Malfoy had slowly been regaining her bearings and mind. She had wandered about Lestrange Manor almost constantly lost in her own thoughts. Her sister had been kind to house her; Bella's presence alone guaranteed the new widow a great deal of rather violent protection from pursuers. Through all of her mourning, though, the woman was coming around- the agony did not subside at all, but the insanity did, and she began to have additional concerns other than the loss of her husband- all still centering around herself, of course.
One of these concerns was that with the recent attack on Upper Slaughter, the Death Eaters could not stay where they were. She was NOT a Death Eater, but she was rather stuck migrating with them, was she not? Rodolphus had certainly informed her of nothing (she wondered how her husband tolerated such neglect). Thus, perhaps it was not entirely accidental when Narcissa was in the right hallway at the right time to hear the tail end of a conversation between Roddy and her sister.
Packing?! Were they just going to leave her here?? Rubbish. Waiting unseen until Rodolphus was out of view, Narcissa crept into her sister's quarters, uninvited. "Sister, while I do appreciate your allowing me my space and peace in light of my husband's murder, I do believe we have certain matters to discuss. Is now an opportune time?"
Bellatrix was now the only one Narcissa would act even remotely subservient toward, since Andromeda had taken away the other.