The 50-year-old woman, Angela Glover, ran a dog charity in Tonga before the pacific nation was hit by a tsunami triggered by a massive underwater volcanic eruption on Saturday.
Her brother, Nick Eleini, said the family was "shattered" at the loss of the "emotional heart of our family."
The eruption of Hunga-Tonga-Hunga-Ha'apai volcano, which sits on the seismically active Pacific Ring of Fire, sent tsunami waves across the Pacific Ocean and was heard some 1,430 miles away in New Zealand.
Internet and telephone communications on the South Pacific island archipelago are extremely limited and outlying coastal areas remain cut off.
But initial reports suggested no mass casualties on Tonga's main island of Tongatapu, though two people were reported missing and the capital Nuku'alofa was badly damaged, the United Nations said on Monday.