About a lord who is tricked into marrying a different sister

[Ashley]

Book starts out in a dimly lit chapel so the lord doesnt know that the sister he is suppose to marry isnt the one there, the next day after he and her do it, he is now forced to stay with her even though he was tricked, eventually they end up falling in love with each other and i think he either has a previous child with another or he is raises neice or nephew. I cannot remeber the name of the book but i believe it has a lady with a green dress and a castle or tower behind her. Please help!