What does it mean to be sisters?

The first answer that comes to mind is, of course, blood-related female siblings.
Following that, another answer is a term used to refer to female devotees in a religious context.

However, it seems that neither of these cases applies to Lief’s mother and Costin.

Costin is a dragonkin, while Lief’s mother is human.
Alchemists are individuals who seek to change the world according to their will, so their religious beliefs tend to be weak.
Rather than being atheists, they are closer to being non-religious, with the few who believe in …