Historical romance title/author

I don't know if you can help but I'm trying to find the title/author of a book I read a while ago, the plot centered around an American lady (I believe in the first chapter she sets a curtain on fire) marrying a lord who is believed to have killed his first wife. The hero has a son who does not talk, and the main plot surrounded the mystery of the first wife and someone who is trying to kill the hero and heroine. I believe that it was the wife's lover who killed her and the son witnessed it which is why he doesn't talk. Also the heroine involves her husbands former mistress to help her uncover who is trying to kill her husband. I believe the heroines cousin is called Charlotte and that she has her own book.