In general, she knew the First Born's story because she had deciphered the tattoo on the corpse of the witch who had predicted his destiny. The witch had prophetic powers, so it makes sense that she could have predicted the FB's return.
I guess "she deciphered a witch's full body tattoo" isn't the gold standard of plauibility in most contexts. But I think it works well enough in this mythological/fantasy corner of the universe.