You did it! We figured this one out! It is a truth universally acknowledged (by me for certain) that the Bitchery pretty much knows everything, and really, it's true. Scroll down to see the solution for this HaBO - and many thanks!

This HaBO is from Moa, who desperately wants to find this story. Spoilers for the ending:
The book I’m looking for is actually a novella or similar. I think it was part of an anthology.
The world is split into two dimensions and in the other dimension, there are demons and angels. The heroine went to the other dimension with her sister years prior; the sister ended up disappearing. The heroine is back to search for her sister. The hero is some sort of lord over the little town close by the entrance to the other dimension. He had the hots for the heroine three years ago, but didn’t make a move since she was too young. He likes her, she likes him, but doesn’t trust him etc etc.
In the end she discovers that he is an angel who comes from a dimension where only angels can enter or open up the gates. He takes her there, they have hot sex in a giant bathtub, they take the hero’s mother away from the dimension, and punish those who have been trafficking humans.
As far as I remember that sex was HOT! And I need it.
That seems like a lot happening for a novella-length romance!

I had the answer hours ago, but I didn’t have access to a keyboard so it had to wait…. I believe it is:
Night’s Darkest Embrace by Jeaniene Frost
Enter the dark realm of Nocturna in Jeaniene Frost’s redhot tale, where blisteringly sexy Raphael dominates the demons of a lawless dimension and tries to help a beautiful young woman avenge her cousin’s disappearance. Originally appeared in the Haunted By Your Touch anthology.
Everything fits, I’ve read it a few times myself, except it is her cousin and not her sister. I read it when it came out in the anthology, but I think last year she released it by itself.
@llaph- God, thank you! I’ve been trying to hunt that down all day! I’ve read that story more than once, but couldn’t find the collection it’s in, and couldn’t decide between Frost, Showalter, or Kenyon as the author.
Now that you’ve pinpointed it, of course, I remember everything, lol.
Available for purchase for $1.99, although personally, I object to spending that much for only 131 pages
https://www.amazon.com/Nights-Darkest-Embrace-Jeaniene-Frost-ebook/dp/B007QUU9JO/ref=sr_1_1?ie=UTF8&qid=1501640120&sr=8-1&keywords=Night%E2%80%99s+Darkest+Embrace
Which is kinda a strange price considering the entire anthology is available for the same price.
https://www.amazon.com/Haunted-Your-Touch-Doomsday-Brethren-ebook/dp/B003VM8Q8C/ref=sr_1_fkmr0_1?s=digital-text&ie=UTF8&qid=1501640204&sr=1-1-fkmr0&keywords=Haunted+By+Your+Touch+anthology
You’re very welcome. I remember the first time I read it I thought it was pretty different from what I’ve read of hers before. Which is probably why it stuck so well in my head, lol. As soon as I saw the title I was like, “oh, oh, oh, I KNOW that book!”
The other stories in that anthology were okay ( in my opinion anyway) but since I’m cheap I like to get my pages for my money so I would’ve bought the anthology if I didn’t already have the story, heh.
Definitely Night’s Darkest Embrace… I’m fond of this particular novella. Damn, the one time I know a HABO, I don’t check until the end of the day 🙂
Oh my god, thank you so so much!!! This has been bothering me för ages. I’m so happy.