The pair, named Patricia and Alison respectively, are said to have died on the same day, although the causes of death are not yet known.
"My heart is broken that I've lost my mother this past weekend," Carey said in a statement to People magazine.
"Sadly, in a tragic turn of events, my sister lost her life on the same day.
"I feel blessed that I was able to spend the last week with my mom before she passed.
"I appreciate everyone's love and support and respect for my privacy during this impossible time."
Carey's mother Patricia and father Alfred separated when she was three.
In 2020, the pop singer wrote about the "complicated" nature of the relationship she had with her mother in her memoir, The Meaning Of Mariah Carey.
Carey did maintain a relationship with her opera singer-trained mother, recording a festive duet of O Come All Ye Faithful and Hallelujah Chorus in 2010.
She dedicated her memoir in part to her mother, writing: "And to Pat, my mother, who, through it all, I do believe actually did the best she could. I will love you the best I can, always."
Carey's relationship with sister Alison also appeared to be complex.
In the memoir, she described it as "emotionally and physically safer for me not to have any contact" with Alison or brother Morgan.
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