The Alchemy of Moonlight by David Ferraro
When Emile's aunt declares he must marry or be disowned for being gay-he runs away to hide as a servant in Count Montoni's mansion. In their service, Emile tends to the family who all suffer a strange affliction on the full moon. And after overhearing suspicious family arguments, he finds a body on the estate, drawing the attention of a handsome doctor as well as the count's charismatic nephew, Henri. Before Emile can sort out his affections or unravel the growing Montoni family mystery, his identity is revealed and his aunt comes to collect him. When she arrives, Count Montoni forces everyone to depart to the remote Udolpho Castle-where there are no witnesses and no chance for escape. There, Emile realizes that he will have to risk his life to find the love he deserves-and survive the Montoni family.