TMZ obtained a copy of the document which lists Karen's official cause of death as cardiac arrest and sudden cardiac arrhythmia. It also notes Type 2 diabetes as a secondary underlying cause.

The docs also say Karen had bipolar disorder, and was cremated following her death on March 19. She was 65.

TMZ broke the story ... fire officials were already attempting lifesaving measures when deputies arrived at Karen's home in San Marcos, CA, but they could not revive her.

Karen was Kris' only sister, born 3 years after the famed momager.

The sisters grew up together in San Diego, living mostly with their mom, MJ, following her divorce from their father, Robert Houghton.

Sources with direct knowledge told TMZ ... Kris was extremely upset by the loss of her sister -- and also paid a heartfelt IG tribute to Karen, reflecting on their history and relationship.

Despite their sometimes complicated relationship, marked by Karen alleging in 2014 that Kris' fame had changed her, Kris' tribute was heartfelt.