Vesper or Cahill?

Prologue

After wondering for so long, Amy and Dan Cahill found out what branch they really belong to; The Madrigals. Their mother, Hope, and their grandmother, Grace, were Madrigals as well. They knew that they were Cahills because of their mom's side. But what about Arthur Trent?

Every time Arthur was just referred to as Hope's partner in the hunt. Other than that he was a teacher, Amy and Dan didn't know much about him.

It wasn't a coincidence that Arthur was working in Turkey when Hope was. The Vespers had given him a mission. To get Hope to fall in love with him. He didn't know, at the time, everything about her. He fell in love with her. In Turkey, Arthur learned some terrifying information about the Vespers. He realized that he needed to cut all ties with the Vespers. Before Arthur proposed to Hope, he told her all about his family background. Of course, Hope already knew everything because Mr. McIntyre's spies have been following Arthur for months.

So there. They get married, have two kids, and then they die in an "accidental" fire. Now what? Their kids were young so they could never tell them anything. Amy and Dan found out that their mother was a Madrigal so they are too, right? Wrong? Arthur and Hope decided that Dan would be a Madrigals and a Cahill like Hope. Amy, on the other hand…