Blind Assumptions

Her plan was to prepare any random dress, but Rowan told her that since everyone was focused on Lorelei's birthday, no one would notice if something went wrong with Sienna's dress. But if anything did go wrong, the Kensington family would lose face.

Since Sienna and Lorelei had similar builds, they could just give Sienna one of Lorelei's old dresses. That would be safer and less of a hassle.

Cecil