This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / I Miss You I Miss You
The real life backstory.
Right after Cilla gets hit by the car, Tina is in shock. A man comes to her and wants to drive her home. While walking slowly away, she is muttering apathetically " Cilla is just hit by a car, she is just hit by a car and not ran over by a car. It's not so severe, is it?"
Shortly later, when Tina drives together with her mother back to the accident scene and the mother runs crying toward Cilla who gets treated by Paramedics.
Among her other cries, she screams that she is Cilla's mother. The twins' birth mother died when they were toddlers and Albert married Monika a few years later, so she is technically Cilla's stepmother. The story focuses a lot on how the person Tina loses is her identical twin, a person who, genetically, is as close to her as could possibly be. It's heartwarming, but also very tearjerking, that they also point out that genetics aren't everything, and that Monika is Cilla's true mother, despite not having given birth to her.
When Tina rings the school to tell that Cilla is dead, still in shock:
Tina: Hello, this is Tina Nigliolo, 8b. We can't come today, Cilla and I.
Secretary: Don't you feel well?
Tina: No, I am fine, but Cilla...
Tina: Cilla is dead, I think. So we can't come today.
Secretary: What?...Where are you?
Tina: There is no danger with me. Cilla is going to be fine, she is just hit by a car. Mom is here.
When Tina and her father are hugging in the hospital shortly after Cilla died, both crying. A nurse comes to them and tells that they can look at Cilla now. Tina screams that she never wants to look and runs away, crying.
Some other scenes that take place in the hospital
"Who is going to sleep with me in the room tonight?"
Tina sleeps together with her parents in their bed the night after Cilla died. While Tina dreams a rather happy dream about her sister and her playing on a beach, her mother lies beside her and is sobbing quietly.
The part where Georg, the school's social worker, helps Tina to her first real breakthrough in the grief process. Tina's monologue in that scene is heartbreaking.
Tina: The only thing I can't forgive her for is that she died. I just don't understand how she could do it. How the hell can she go and get hit by a car and die and leave me behind to take care of Mom and Dad and friends and life and missing her, missing her, I miss you, I miss you so god damned much!
Tina playing her violin for the first time after Cilla's death and in her mind hearing Cilla join in on her flute. When she stops playing she notices her father listening at the doorway, moved to tears.
"I'll never be able to hold you again, Cilla."
Some of the dreams Tina have after Cilla's death, particularly the ones where a dead Cilla crawls into bed with her and wants Tina to help her get warm again.
In the book Tina opens a spread in her diary and fills one page with Cilla's name and the other with "I miss you". In the movie she writes "I miss you" over and over again in a Word document.