Something I'm always interested in reading about, and writing. "Sore tooth" in the sense of you keep working at it, even if it's slightly painful (or at least I do that). There was a meme on DW last year that talked about "literary kinks", which I think also works as a term.

I feel like sibling relationships in general, but especially sister relationships and brother-sister relationships, are hugely underexplored in most media--I think the family you find is way more of a common theme, or brother relationships (I'm looking at you, FMA, Supernatural).