What are the motifs in Sister: A Novel by Rosamund Lupton?

Perseverance is an important idea. Although many characters in this book give in to what is easiest to believe regarding Tess' death, there is one character who perseveres to the end, Bee. She does not accept the police's beliefs that Tess committed suicide. She sticks to this belief in her sister's love of life until she learns the identity of the murderer. She does not allow others to dissuade her, although many try. She understands they believe she is mad with grief, but she refuses to give in. In the end, she learns the truth of what happened to her sister.