The pair who are named Patricia, who was Irish American, and Alison respectively, died on the same day. Their causes of death have yet to be announced.
Carey's mother Patricia had separated from her father Alfred when she was three years old.
Mariah Carey confirmed this news regarding their deaths in a statement shared via the PA news agency.
"My heart is broken that I’ve lost my mother this past weekend", the singer said. "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", she added. "I appreciate everyone’s love and support and respect for my privacy during this impossible time".
Mariah Carey had previously written about the "complicated" relationship that she had with her mother in her memoir, The Meaning Of Mariah Carey.
The singer still maintained a relationship with her mother, even recording a duet O Come On All Ye Faithful and Hallelujah chorus in 2010.
Dedicating her memoir in part to Patricia, Mariah Carey wrote: "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".
She also detailed her complex relationship with her sister Alison and brother Morgan, saying it was "emotionally and physically safer for me not to have any contact".
