<P> Cristina and Meredith repair their relationship when Meredith confesses that Cristina was correct, her skills have surpassed Meredith's . Cristina moves to Switzerland take up a job offer from Dr. Preston Burke, her former attending, mentor and fiancé, who was looking for a replacement at a research hospital he was running, thus leaving Alex in charge of being Meredith's "person" in her place, an honor he gladly accepts . </P> <P> Meredith and Derek's marriage becomes strained when Derek goes against his promise and accepts an offer from the U.S. President to participate in the Brain Mapping Initiative, which consumes his time and eclipses Meredith, who is feeling increasingly left behind and mommy - tracked . He receives an offer to head the project itself in Washington D.C., meaning that he would have to be based there permanently . Meredith puts her foot down as she does not want to uproot their young family to move across the country for his career at the sacrifice of her own . They begin a series of on - and - off arguments and "cold wars" over their careers . Derek accepts the job in the heat of the moment and promptly leaves for Washington . During a phone call with Meredith, they agree to work things out after she tells him that she did not want them to become "one of those couples" and he reciprocates, saying that he missed her . She privately admits to Alex that she has realized that she could live independently of Derek, but chooses not to . </P> <P> Meredith finds out she has a maternal half sister named Maggie Pierce who is now working in Grey Sloan Memorial . Meredith is in denial and rejects Maggie, thinking she would have remembered if her mother was pregnant until she finds a hospital document confirming the revelation . Meredith tries to piece together her relationship with her mother and half sister by going through old videos of her mother . She eventually recovers her repressed memories of the pregnancy when she views her mother's diary and has a change of heart, choosing to accept Maggie and begin building a relationship . </P> <P> Meredith is widowed when Derek is killed in a car accident and was taken to an understaffed hospital . The doctors failed to recognize his head injury in time and allowed personal conflicts to interfere . Derek is declared brain dead, and Meredith must go to the medical center to consent to remove him from life support, shortly before she's hit with the first waves of morning sickness . She tells Penny, the intern who was assigned to Derek that every doctor has "that one" patient who dies on their watch and haunts them forever and "that one will make you work harder, and they make you better ." </P>

How are dr grey and dr pierce related