There Are Plenty More Dukes in the Sea by Samantha Holt

The Templeton siblings have inherited a fortune. But there’s a catch…
I, George Lockett, do decree that my grandchildren, herein named, will inherit my fortune in its entirety, so long as they fulfill the following terms. Each person must complete their task to my lawyer’s satisfaction or their share of the inheritance shall be forfeit.
There are many reasons Lady Angel Templeton needs her grandfather’s money. Of course she wants to be less of a burden to her oldest brother but, goodness, once she has even more money, she can have her pick of the men! A certain Duke of Norwick would be a fool to ignore her…
Unfortunately, their beloved grandfather decided not to make it easy on the Templetons, though Angel is convinced her task must be the hardest. To fulfill the terms of the will, she must perform unpaid duties as a lady’s companion for two months to the entirely unmanageable Mrs. Stone.
Two whole months of work.
What’s worse, Mrs. Stone is determined to be an even bigger handful than Angel herself.
No…what’s worse, is the lady comes along with a miserable wretch of a nephew. While she cannot deny he is handsome in that dark, brooding way so many women adore, Angel does not enjoy the company of serious men–especially when they have already declared they do not trust her one jot.
Well, she doesn’t trust him either. It soon becomes obvious why the stern Mr. Hunter is determined to haunt her every move–he must want Mrs. Stone’s money!
Well, Angel cannot allow anything happen to her new friend, especially when she risks losing everything.