She had never invited her she realized. Nellie had never turned to Bellatrix and offered for her to stay. Likewise, Bellatrix had never actually asked. She had just declared it one morning in th bakehouse and Nellie followed along and began hatching a plan of her own to help Bellatrix succeed in her s.
She had done so before, she realized, with Sweeney. Once in a belligerent rag had had decided he wanted to kill every man in London, and Nellie started thinking on how to make it work and work for her.
She had a reason for doing so with Sweeney though. When Sweeney declared and she set to doing, it was out of a devoted love and dire desperation. But when she did the same with Bellatrix, she was well-off financially and had the man she had spent most of her life dreaming about almost within her grasp.
Maybe it was the delirium of the moment; the inhibition of only just waking up from a wild romp with a warrior from the past and a witch from the future just showing up in her room to fuck her senseless.
And senseless she had apparently stayed as months later she happily housed two insane murderers intent on killing as many as they possibly needed (all so that hey may attain some twisted ideal world that they would probably never be content with.)
Nellie was content though. In all of the house's brooding, blood, and bones Nellie Lovett was content. She had her boy and her bustling business. She had the man that she loved in her home, and a beautiful friend in her bed.
Nellie Lovett didn't know why exactly she went along with these beautiful maniacs and their murderous plans so easily. But truth be told she didn't exactly care.
Humanity and be damned. Sweeney and Bellatrix could stay in her house plotting its destruction forever for all she cared, because Nellie Lovett was happy.