Jesse Robinson said his sibling died last month of unspecified causes, adding that McCarthy had previously suffered from diabetes and a pancreatic illness, The Boston Globe reported Sunday.
McCarthy's Jan. 27 death came just as the rocker was preparing to begin work on the first Post Mortem album in years.
Post Mortem guitarist John Alexander remembered McCarthy as an individual open to all kinds of music, an approach that led to conflicts with band mates like drummer Rick McIver.
"Rick's lyrics were very graphic, gore kind of stuff," he said. "One of the struggles John had with Post Mortem was singing the gore lyrics. They didn't resonate with him, but when you're dealing with other individuals, you have to compromise."
The Globe said beside Robinson, McCarthy is survived by his son Matthew, his mother, father, step-father, another step-brother, two step-sisters and a granddaughter.