Adding Up to Love by Ginny B. Moore

When Alex asked for Fern’s help courting her twin, he agreed to help her get into Oxford, but he didn’t expect to fall in love with the wrong sister.

Fern loved nothing more than math. Relating more to numbers than to people, she dreamed of becoming a mathematician at Oxford University, an impossible path for the youngest daughter of a powerful aristocrat. After living her life in her sister Rose’s shadow, a chance encounter with a handsome student gives her the opportunity to chase her dream. Falling in love with Alex was not part of the equation.
Alex Carroway did not believe in love at first sight until he met his perfect match at a masquerade. Marrying Rose would be the smart choice and give him the future he always dreamed of — if only he hadn’t fallen for her quirky, brilliant sister. With his plans in jeopardy, Alex realizes that doing what is smart is not always what is best.
Now he can’t stop thinking about the woman he wasn’t supposed to notice, and their lives together won’t add up. Will a future with Alex be the one problem Fern cannot solve?