Sabrina, who lives with her mother in Chingford, wants to be a forensic scientist. Her academic progress is all the more impressive when you know that she is caring for her mother, who is seriously ill with cancer and needs help with washing, dressing and getting to the bathroom.
She is one of the thousands of cancer sufferers who receive help in their own homes from more than 5,000 Marie Curie Cancer Care nurses. Patients are put in touch with the service, which is provided free, by their district nurse or GP.
Mrs Costello says: "I have a Marie Curie nurse between 10pm and 7am three nights a week, which is an enormous help to me and Sabrina. It means that Sabrina can get a good night's sleep. When it's just the two of us here, I know that Sabrina is listening out for every move I make."
Sabrina agrees. "It's been much easier since the Marie Curie nurses have been coming. It was getting difficult because recently Mum has not been sleeping through the night. But when they come it gives me a break."
Mrs Costello, now 40, was only 33 when she discovered a lump in her breast. Cancer was diagnosed. Two years later she developed cancer in her other breast, and the disease spread to her brain and back. As a result she suffers from epileptic fits and weakness down her left side.
Marie Curie nurses, who also spend time with her during the day, help her to take her medication and see to her daily needs. They also make sure that her symptoms are under control and that her pain relief is adequate. "You know you are safe when they are there," she says. "And when you are feeling rough, there's someone there to reassure you."
Pat Unger, one of the Marie Curie nurses helping to care for Mrs Costello, says the ethos of the charity's nurses is to complement the work of the district nurses in promoting and maintaining a good quality of life for individual patients living at home with cancer. "To be effective, this must acknowledge the needs of family and other carers."
The role of the Marie Curie nurse will vary according to the patient's needs, Miss Unger says. "The one-to-one relationship that often builds up with patients allows time for them to talk freely and express feelings that are a burden to them - which can bring about a sense of peace."
She describes Mrs Costello as a cheerful and determined person whose main concern is the welfare of her daughter, and she adds: "Without support from Marie Curie Cancer Care, Mrs Costello would have to spend longer periods in hospital. And it's very important to her that Sabrina receives the help she needs in order to cope, particularly at this time when she is in the middle of studying for her exams."
Mrs Costello says Sabrina's perseverance with her studies is one of the things that helps to keep her going. She is keeping her fingers crossed that her daughter will - with the help of Marie Curie nurses - pass all nine GCSEs she plans to sit.