A few months ago, Smexy Books had a post about the recent popularity of step-sibling romances. The author found it somewhat perplexing, since she expected to find books breaking taboos and instead found a book where the adult protagonists were strangers when their respective parents married.
I've been thinking about the reasons for the appeal of such books. I don't think it is necessarily because readers are looking for quasi-incest fantasies. I think they are looking for close-knit family fantasies.
Historical romances are less likely to feature the step-sibling trope (possibly because marriages were less fluid in prior eras). However, the common tropes of Brother's Best Friend and Childhood Friend are functionally similar. Usually one protagonist (usually the hero) comes from a broken home and the other (usually the heroine) has a loving family. A marriage between them gives the de facto orphan an official place in the family he already admires/envies.
Last Night's Scandal by Loretta Chase. We first met the protagonists as children in Chase's third Carsington Brothers book, Lord Perfect. Peregrine's irresponsible parents essentially abandoned him to the care of servants and distant relatives, and Benedict Carsington became the father figure he never had. Benedict's stepdaughter, Olivia, was Peregrine's childhood partner in crime. After several years apart, they meet again as adults, and sparks fly.
Tessa Dare's Goddess of the Hunt is a very charming example of the Brother's Best Friend trope. Lucy asks Jeremy to coach her in seductive wiles, so she may win her longtime crush. He is reluctant, but he fears she might ask someone less honorable for assistance (a common justification for overcoming such scruples in romance novels), so he agrees. He gradually sees her not as a little-sister type
Caroline Linden's Love and Other Scandals is a different type of Brother's Best Friend romance. In this case, Joan Bennett thoroughly disapproves of her brother's debauched friend Tristin Burke. The set-up has a little bit in common with Loretta Chase's Lord of Scoundrels (which is really in a class all by itself). Linden's story is far less outrageous, but it is a satisfying enemies-to-lovers story with a family connection.
I Loved a Rogue. As the conclusion to her Prince Catchers series, it solves the mystery of the Caulfield sisters' parentage. While it seems to be a Childhood Friend romance, from the very beginning I felt like the protagonists were de facto step-siblings. Taliesin was not related by blood nor adoption to Elinor's adoptive father, but he spent his boyhood summers as their live-in servant. He was basically a male Cinderella (cleaning the ashes from the hearth was specifically one of his chores). He and Elinor go on a quest to find her birth family, and he discovers secrets about his own background that he never suspected. In the end, the mentor who once sent him away (which was not intended as a rejection, although Taliesin always perceived it as such) becomes his father-in-law. I'm sure that won't make family Christmas gatherings at all awkward.