Quoc: 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.

Answer this question based on the passage: how are dr grey and dr pierce related?
HW Chung: OK, some thoughts:
The relevant information is: Meredith finds out she has a maternal half sister named Maggie Pierce who is now working in Grey Sloan Memorial. The answer is a maternal half sister.