Vampires are creatures of chaos, so the rules governing their magic is more flexible than for creatures of order like, say, demons. As I had Lemma say a couple of times, te stronger they are, the more they can bend the rules.
So, 1) an inn is sort of like a home–traditionally, the innkeeper and family live there–and sort of not. That “sort of not” creates some wiggle room. 2) The boy opening the door could be interpreted as permission, creating some more wiggle room. 3) Brea is very, very powerful, so she can fully exploit any wiggle room she’s given.
Thanks for writing, I’m glad you enjoyed it enough for a reread!