So I got out Molly's old footies and coat, which she used only a few times, and Brea let me put them on her without a struggle. Siri was the one who was difficult, constantly getting her nose in the way while I was trying to put the booties on Brea. Finally we were ready. I think they helped.
Siri doesn't seem to be bothered by the cold as much, but she has a heavier coat, and supposedly, she's part husky.