What would happen if Nellie was actually Amy and Dan's sister and no one but Grace and Fiske knows.
Arthur Trent and Hope Cahill were over the moon with the birth of their first child, she was a bright happy little girl who they named Nellie. However after a year of having Nellie, they realized that their love for her was just too strong. Because of the Cahills and their terrible ways, Arthur and Hope knew that their little girl would never have a normal life. So when Nellie turned four they took her to an orphanage and left her forever.
Four years later Hope found out she was pregnant and gave birth to another baby girl whom they named Amy. This time they kept the child even though they knew it would endanger her because leaving their first child was the worst decision they ever made. Three years later they had a baby boy called Dan and the were a happy family. However one night when Dan was three and Amy was six, Arthur and Hope tragically died in a house fire but Dan and Amy survived. Meanwhile, Nellie had been adopted by a very loving and kind couple who became her adoptive parents. Nellie believed they were her real parents who came back to get her because when Nellie was left by her parents, all she remembers was her parents hugging her and saying that they loved her, but she never remembers their faces. When she was fourteen her parents told her that she was adopted and that her real parents died last night but her little brother and sister survived, however she was not allowed to attend the funeral or meet her siblings. Nellie was confused but she still loved her adoptive parents. Nellies father started getting her into flying soon after and her mother ensured Nellie could speak more than five languages fluently. When she turned 16 her father also made sure she could drive very well and enrolled her in defensive driving classes. Nellie had a feeling that all these classes were not just for her fun but maybe for something else. Years later when she was 22, Nellie went for a job interview which her father had recommended to her. It was to be an au pair for a fourteen year old girl named Amy and her eleven year old brother named Dan. For the interview she met with their grandmother Grace and that's when everything started to make sense. Grace told her that she was a good friend of Nellie's father which is why he recommended the job, she went on to explain that she was getting quite sick and would die in a matter of months. She said that when she died a contest would begin called the 39 clues which would be run by Grace's lawyer William McIntyre. Grace explained all about the Cahills and soon Nellie understood the clue hunt and Grace also told her when she died that she would refer to McIntyre and her brother Fiske. She said the reason Nellie learnt to drive well, speak more than ten languages and fly a plane was because she would act as their guardian during the hunt and have to protect them. Grace also said that she must get close to them so they would go to her and she would have to act surprised. If she accepted she would be given one million dollars up front and if she went the whole clue hunt without telling them she would get another 500,000 dollars. Nellie accepted and so began the search for the 39 clues as we know it.
"Amy come on, Fiske wants us and Nellie for a special talk now", Amy's 11 year old brother was yelling at her from down stairs. "Coming", she yelled back at him. They quickly ran to meet Fiske in the Cahill Command Centre. "Ah Amy, Dan there you are sit down and we'll have a little talk", greeted Fiske. "Hey kiddos", Nellie also greeted. "Now you are both aware that Nellie was adopted as a kid and she was trained especially to come and supervise you guys during the clue hunt". Fiske continued. He received two nods and continued. "Well something I'm sure you didn't know and something that Nellie also didn't know is that her real parents names were Arthur Trent and Hope Cahill, which is why she was trained for and given this job." He then received three confused and shocked faces, so he continued. "If you haven't realized by now then I guess I should tell you that the three of you are actually siblings, so Amy and Dan meet your big sister and Nellie meet your little brother and sister". The three of them turned to each other had Nellie squeezed them in a tight hug and said, "well I guess your stuck with me now kiddos."