The Friends star and her half-brother, John Melick, confirmed the news, sharing that their mother "passed peacefully, surrounded by family and and friends after enduring a long illness," in a statement released to ET.
"We ask that our family’s privacy be respected as we grieve our loss," they continued.
Dow had been in poor health since 2011, when she suffered a stroke, followed by another the next year, People reports. The former actress was married to Aniston's father, soap opera star John Aniston, from 1965 to 1980. She was previously married to Melick's father, Jack Melick, from 1956 to 1961.