Sebastian was born in Basseterre, Saint Kitts and Nevis (then part of the United Kingdom), on 21 September 1913, as the first of twelve children born to Joseph Matthew Sebastian (1891–1944) and Inez Veronica Hodge. Her father was a Caribbean trade union leader and politician. From her father, she had several younger half-siblings.

At the age of 5, she enrolled at Miss Walters Private School. Later, she transitioned to the Girls School, where she served as a student teacher, instructing in math for over 3 years. At the age of 15, she earned the Moravian Scholarship, which allowed her to attend the Teachers’ Training College in Antigua for a duration of two years. During her time at the college, she became involved in the Guiding Movement (Girl Scouts) as a Ranger, a program designed for individuals aged 14 to 18. She continued her involvement with the movement upon her return to Saint Kitts. Following her graduation, she came back to Saint Kitts to pursue her teaching career further.

Sebastian never married nor had children. From 1932 to 1941, she served as an assistant teacher at the Basseterre Boys’ School. In 1941, she assumed the role of Headmistress at the Dieppe Bay Infant (Primary) School. When she began her tenure, the school had only three classes. She continued to lead the school for 25 years until her transfer to Irish Town School, where she stayed until her retirement in September 1969.

Following her mother’s passing, she left Saint Kitts to live in Nova Scotia, Canada with her sister, Dorette, where she would spend the next 27 years. Around 2000, she moved to Queens, New York, to live with her sister Elise. Then, in 2005, she moved to Barryville, and finally, in 2011, she settled in Philadelphia. At the age of 100, she had a pacemaker implanted.

In 1995, Bernice’s brother, Cuthbert Sebastian, was appointed Governor-General of Saint Kitts and Nevis. On her 109th birthday, she had one living sister and two living half-sisters. Her half-sister, Stella Eudora Shaw (who passed away in February 2023, aged 91) was the First Lady of Dominica between 1998 and 2003.