Man, though, I want to talk about My Sister's Mistressby Jessica X and Cartesian Planeswalker for a minute because I fucking adore it, and I need to give it some props.
If it were simply a really good, smutty kink fic, that would already be worth celebrating, but it's more than that: while this is a fic where the kinks are varied, hot, wonderfully well-depicted, and present throughout the story, it's also a phenomenal character study for both Elsa and Anna.
This is because all that smut (while still being enjoyable and sexy on its own) really serves to illuminate character, as Elsa and Anna allow their numerous, complicated emotions to manifest in the things they do to each other while concealed in Anna's soundproofed bedroom. I'm often not a big fan of plots where one of them has to marry a foreign nobleman for the good of the realm, but I love it here. I love how the heightened emotions of the scenario push them both forward, and I love how honest it feels in its exploration of their thoughts and feelings. The authors have done a great job of rooting their story in the canon versions of the characters, so even (and perhaps especially) when they end up in extreme situations, the characterizations are the thing that shine through the brightest. It's so good it makes me weak in the knees, and as long as you have a stomach for the stuff it depicts, I can't recommend it highly enough.
(NOTE: This is a kink fic,and some of it could be extremely triggering. However, the authors have done a great job of extensively detailing what kinks you'll find in each chapter, so if something does sound like it would be too much for you, it's easily skippable.)