I don't know what I missed; I think there is no chemistry between Sebastian and Mary. I have the impression that they were just childhood friends and there is nothing more. There is no pull or yearning. Sebastian was bent on retribution, the plots about bringing his uncle down were kind of boring. The romantic relationship was lacking for me. I didn't feel that either of them wanted to be with each other or was fighting attraction. Very odd feelings.
The 3 brothers running from an evil uncle trope is interesting enough. I find the relationship among the brothers more interesting than that between Sebastian and Mary. They just have very little together other than childhood friendship. Sebastian displayed some standard jealousy and Mary did all she could to help him for whatever reason. Gratitude on Sebastian's side and loyalty on Mary's side but there is no unrequitted love or passion. Just 2 people who have known each other for a long time and ended up together. For a tortured hero like Sebastian, his character is surprisingly boring. Mary is bland. She is not feisty, she is not graceful or soft-hearted, she is just....Mary, who helped the brothers escape when she was 12 and a loyal friend of Sebastian, the duke.
I am however very interested in the other 2 brothers. Sometimes the first book in a series suffers because the writer spends too much time building the series in the first book and there book loses its own appeal. On to Tristan next!