Audrina Patridge, star of ‘The Hills,’ is mourning the death of her 15-year-old niece; the cause of death has not been revealed.

Audrina Patridge, who rose to prominence in The Hills for six seasons between 2006 and 2010, recently revealed that her 15-year-old niece, Sadie Loza, passed away.

The adolescent girl was born in 2008 to Casey Loza, Audrina’s sister, and Aaron Hershman, whom Casey previously dated.

Audrina paid tribute to her late niece on Instagram. “I know it’s not goodbye forever, but it is the hardest to say goodbye for now. We will miss you and treasure every moment we had with you. Rest in peace!! We will love you forever and ever!” she wrote, adding that it “hurts even to write this.”