Disclaimer: I own nothing, not even the computer I'm using to post this.
Drusilla was getting bored of plotting the end of the world with Miss Edith. She didn't really want to go through with it, because if the world was gone then the stars would have nowhere to shine, but mainly because her kitten and her jewel would be upset.
And their little friends would be too, but she didn't really care about them. She cared about her Spike and her daddy, and her kitten and her jewel, and even Miss Edith, but she was, afterall, a vampire.
She didn't know how to care about anyone other than those already mentioned.
Finally, Miss Edith decided to sleep. Drusilla left the doll in the crib she'd found in Brazil, and slipped out of the crypt and into the dark Christmas night of Sunnydale. She always visited her kitten at Christmas, and Spike had always assumed it was a 'catholic' thing.
Spike had been sweet, but he wasn't the smartest vampire in the world. He had too much of William in him to realize that fidelity was unusual in vampires, while she had too much of the original Drusilla in her to kill her own flesh and blood like her daddy had.
The stars told her Spike was in town, but she hadn't seen him. He was unimportant anyway.