I believe that the person in this 1930 census is your Lillian:
Lillian Riesner
United States Census, 1930
Name: Lillian Riesner
Event Type: Census
Event Year: 1930
Event Place: Brooklyn (Districts 1251-1500), Kings, New York, United States
Gender: Female
Age: 12
Marital Status: Single
Race: White
Race (Original): White
Relationship to Head of Household: Stepdaughter
Relationship to Head of Household (Original): Stepdaughter
Birth Year (Estimated): 1918
Birthplace: New York
Father's Birthplace: New York
Mother's Birthplace: Massachusetts
Household Role Gender Age Birthplace
James W Obrien Head M 30 New York
Lillian Obrien Wife F 30 Massachusetts
Irving Riesner Stepson M 14 New York
Lillian Riesner Stepdaughter F 12 New York
https://familysearch.org/ark:/61903/1:1:X4JP-9PV
the Lillian brother family in 1940 census:
Irving Reisner
United States Census, 1940
Name: Irving Reisner
Event Type: Census
Event Date: 1940
Event Place: Assembly District 16, Brooklyn, New York City, Kings, New York, United States
Gender: Male
Age: 25
Marital Status: Married
Race (Original): White
Race: White
Relationship to Head of Household (Original): Head
Relationship to Head of Household: Head
Birthplace: New York
Birth Year (Estimated): 1915
Last Place of Residence: Same House
Household Role Gender Age Birthplace
Irving Reisner Head M 25 New York
Helen Reisner Wife F 22 Norway
Helen Reisner Daughter F 2 New York
James Fortier Foster-son M 3 New York
Lillian O'Brien Mother-in-law F 40 Massachusetts ( step mother)
James O'Brien Father-in-law M 38 New York (step father)
https://familysearch.org/ark:/61903/3:1:3QS7-89MY-59H1
best,
suanj