Big Ang's sister, Janine Detore, tells TMZ ... Linda died Thursday at Staten Island University Hospital, not long after learning she had COVID-19.

We're told Linda, who also had a few cameos on "Mob Wives," was placed on a ventilator after doctors diagnosed her with the deadly virus, but it wasn't enough to help her pull through.

Linda had underlying medical conditions making her more at risk for a fatal case of COVID-19 -- besides her age, we're told she had breast cancer, and recently had surgery to implant a chemo port.

We're told Linda's daughters were taking care of her after the surgery, and when she developed a fever they took her back to the hospital thinking the chemo port was getting infected, and that's when Linda tested positive for COVID.

It's been 5 years since Big Ang passed after battling throat cancer.

Linda's family and friends were keeping the faith, and remained hopeful to the end that she could beat the virus.

She was 67.