Courtney Milan is an auto-buy author for me. Her historical romance novels never fail to draw me into her world and into the lives of her characters. Her characters are never conventional or cliched. They feel like real people with quirks and flaws. I always have a really emotional response to her stories and each one leaves me wanting more.The Heiress Effect is book 2 in the Brothers Sinister series. It focuses on Oliver Marshall, the son of Serena and Hugo from the The Governess Affair, a wonderful short prequel story to the series. You don't need to read The Governess Affair to understand the series but it's fantastic and I highly recommend it. Having been born a "bastard," Oliver is very aware of society's rules and works hard to maintain his reputation in order to be taken seriously and to make a different in Parliament. Jane Fairfield is anything but proper. She's loud and says things that a proper young lady should never say. She is an heiress so society must tolerate her but they all ridicule her behind her back. And this is fine with Jane who wishes to remain unmarried in order to stay with her under age sister and protect her from a pious, overbearing uncle.This is a romance that doesn't start as a romance. Oliver finds Jane ridiculous and totally improper and Jane still holds a grudge against Oliver for a wrong committed years ago and for which he has since made up for. However, their dislike soon becomes friendship when they realize that each is not what they first appear to be. I really liked watching Oliver and Jane get to know each other. From barbs and insults to frank conversations and inside jokes, their relationship slowly evolves and I loved watching them slowly fall for each other.I'll admit that I was not a huge fan of Jane's at first. She seemed a little bit frivolous and selfish. She soon started to grow on me though and I found her to be kind, an extremely protective older sister, smart and fearless. While some of it is an act, Jane will never be what society considers a proper lady. I admire her for being herself even when it means ridicule. Oliver is a great hero. He is honorable and intelligent though he has flaws. He struggles with who he and I think has always felt a little out of place in society. This is why Oliver and Jane make such a great pair though. Neither of them fits the box that society wants to put them in and they make a great team. There is also a great cast of supporting characters. Robert and Minnie from The Duchess War make an appearance as does Sebastian, another Sinister Brother. I loved seeing Oliver's parents, Selena and Hugo again, older and still in love. Jane's sister Emily has a really sweet storyline a well and I enjoyed her parts in the book. Jane's uncle is the "villain" in the novel though he is not all bad. Like all of Milan's character, he is a complex character that is only too human. The Heiress Effect is another wonderful novel by Courtney Milan. Full of great dialogue and nuanced characters, her books are a must for any romance reader. I can't wait for the next book in this series! *I received this book free of charge from the publisher in exchange for an honest review. This, in no way, affected my opinion or review of this book.