Jackie and Luke Harrison are a divorced New York City couple struggling to help their children Anna and Ben be happy. Luke, an attorney, is living with his new girlfriend, Isabel Kelly, a successful fashion photographer several years his junior. Isabel tries hard to make Anna and Ben feel comfortable and happy with her, but Anna rejects her overtures while Ben, who loves Isabel, adds extra complication with his mischievous nature. Isabel behaves with contempt tempered by caution around Jackie, believing she overcompensates for her divorce by spoiling her children.
Jackie, a former publisher turned stay-at-home mom, gives Isabel a cold reception, seeing her as an overly ambitious career woman. She also continues to harbor malice towards Luke. After a long string of arguments, Luke proposes to Isabel, making her Anna and Ben's soon-to-be stepmother, which causes more friction. Jackie is diagnosed with cancer, which is discovered to be terminal. She experiences a range of negative emotions, angry at the woman who she feels was responsible for breaking up her family, and angry that after all of the sacrifices she made for her family, she will never see her children grow up.
Isabel and Anna continue to disagree. Isabel gets the children a golden retriever puppy and Anna says she is allergic to dogs. Isabel apologizes and says that her father didn't tell her that. Anna takes the dog inside, indicating she lied about her allergy. Isabel and Anna begin to bond over painting. Jackie asks Isabel to pick them up from school. Isabel agrees, then asks if she could take Anna to see a rock band that she likes. Jackie declines, saying she is too young to go to a rock concert on a school night. A few weeks later, Jackie surprises Anna with tickets to the same concert and Anna is thrilled. Jackie thanks Isabel for the idea and Isabel tells her to have a good time.
Luke and Jackie tell the kids about the engagement and Anna is furious. Jackie tells Luke and the children about her illness, resulting in Anna storming out. That night Jackie shows that she can be fun by dancing and singing with the kids.
Jackie and Isabel clash repeatedly, largely over Isabel's parenting. Ben goes missing on Isabel's watch and Jackie claims that she has never lost him, which she later admits to be untrue. They establish a shaky truce, as they come to terms that Isabel will soon step into the role of surrogate mother. The two women bond when Isabel reveals her admiration of Jackie's maternal instincts, while Jackie in turn praises Isabel's hipness as a means to connect with Anna. Isabel finally lets her guard down when she tells Jackie her biggest fear is that on Anna's wedding day, all Anna will wish for is her mother's presence. Jackie says her own fear is that Anna will forget her. Jackie explains to Isabel that, while Jackie will always have their past, Isabel will have their future.
The film ends with the family celebrating Christmas, when a bedridden Jackie is visited in her room by Ben and Anna. Individually, Jackie tells her children that though she will die, she will remain with them as long as they remember her. Later that day, Isabel is taking a family portrait of Luke and Jackie with the children. Jackie demonstrates her acceptance of Isabel by inviting her to join them and she does. The women are shown happily in a photo side by side.

Question: Who has to prove themselves to Anna and Ben?
Answer: Isabel, the new girlfriend of their father, Luke