The Baron Takes a Bet by Matilda Madison

Married by proxy, seduced by passion…
Holly Smyth has had an unlucky life. Tragedy seems to be around every corner for her and her younger siblings. So, when the kindly old Baron Winscombe offers to marry her and save her from destitution, she accepts, thankful to finally have some security.
Until the baron dies, and his nephew, Gavin Winscombe, inherits the title.
The quick-witted Gavin is curious to meet his uncle’s deathbed bride, but upon his arrival at Kingston House, he and Holly are informed that she didn’t marry Gavin Wincombe, the elder. She was married to Gavin Wincombe, the younger, by proxy.
Now tasked with annulling their marriage, Holly and Gavin decided to work together, but the longer they live as husband and wife, the more natural it feels.