The family of legendary singer Natalie Cole released information on her cause of death Friday. They said that the “Unforgettable” singer’s cause of death was idiopathic pulmonary arterial hypertension.

Cole, who had suffered a series of health issues before her death, died on Dec. 31. She was 65 years old.

Her family told the Associated Press that, when she had a kidney transplant in 2009, she was diagnosed with the rare lung disease pulmonary arterial hypertension.

“She responded well to pulmonary arterial hypertension-specific agents over many years, during which she performed many concerts world-wide, but eventually succumbed to intractable right heart failure, an outcome that unfortunately commonly occurs in this progressive disorder,” the family’s statement read.

Cole’s sisters, Timolin and Casey, told Entertainment Tonight that they were able to spend Christmas Eve with Cole.

“We were very blessed and thankful to spend that very special day with her that she loved,” they said. “It was her favorite time of year.”

Cole was the daughter of Nat King Cole and won the 1991 Grammy Album of the Year for Unforgettable... With Love, which featured a duet with her famous father, who died in 1965. He other hits include “This Will Be,” “Our Love” and “Inseparable.”