If the phone is in your pocket, against your body, and under your coat, it’s likely not being exposed to the 15 degree temperature, so the cold may not be a factor.
Is there any chance the phone screen is being turned on while it’s in your pocket? (I once found mine in my pocket with the flashlight on! That burned through the battery in a hurry! Warmed it up quite a bit, too.)
Is your brother’s phone the same age as yours? Is it operating with the same apps? An app that is running could cause this kind of battery drain.
Has the issue continued, or was it a one-time thing?