The Mother does not approve of Elriel
This post is a bit tongue in cheek, but I do think SJM gave a cheeky hint to this in Azriel’s bonus chapter.
When Elriel have their almost kiss, the text reads: “just this taste in the dead of the longest night of the year, where only the Mother might witness them” (emphasis added).
Of course, Rhys ends up seeing and putting a stop to it. The fact that the Mother was invoked and the kiss was then aborted by outside factors suggests that “higher powers” put a stop to it. In a literary sense, it’s a way to foreshadow/signal that Elain and Azriel are not meant to be together. It’s similar to when a character in a novel says “may God strike me if I’m wrong” etc and then there is a lightning strike or something like that.
Basically, the Mother saw Elain and Azriel about to kiss and said “not on my watch, babes.” And/or Rhys is Mother, confirmed.