User blog comment:Xhatefate/DxD Updates~/@comment-16281343-20150122080020/@comment-16281343-20150124012204

She's already living in the Hyoudou Residence along with Kuroka as early as Volume 14.

Regarding the No-Bra Faction, my interpretation on what happened is:

Since Elaine is Le Fay's maid, she inspected her room that she shares with Kuroka. The room is a mess and has many random things put in it courtesy of Kuroka and part of the mess is a bra. Kuroka saying she's doesn't need it as she's part of the No-Bra faction and she can use senjutsu to take care of her boobs.

Still, Elaine forced her to clean the room with Koneko helping her sister.