Looking for a book

Im looking for a book thats about A duke/lord arriving to his family home and finding out his "wife" which he never married making changes to it. The lady thinks she'd married to him ( not him per say) but to the lord. Well it was the lord/duke brother that married her to help her out but never told her his name. He married her under his brother's name and left her there for him to take care of her... For the life of me i cant remember the name of that book, its a historical romance book

Hi and welcome to Librarything! You'll probably find the answer you seek in our Name That Book! group, where they specialize in finding lost titles:
https://www.librarything.com/groups/namethatbook

Please be sure to read the group's header for instructions on posting your request