Nikki Hallander was the older sister of Heather Hallander.
History[]
She was the last person to see Heather before she disappeared, but it was later revealed that Nikki murdered Heather by pushing her into the chimney stack of their apartment after she caught her doing drugs.
When Kristen Vucelik masqueraded as Heather, Nikki was suspicious and after learning Kristen's true identity, she attempted to force Kristen to climb up to the chimney stack by showing her what happened to the "real Heather", but the police were able to prevent her from doing that and Nikki was taken into custody for her sister's death. (SVU: "Stranger")