D'Souza also claimed other cruise passengers had been locked out of their rooms on Monday without explanation.
The family believes the rooms were being disinfected.
"We'd like some answers," D'Souza told Sydney Morning Herald.
P&O has denied there was a gastro outbreak, and Queensland Police told Daily Mail Australia Carlson's cause of death was not being treated as suspicious.
"There is no reported gastro outbreak onboard," a P&O spokesperson told Daily Mail Australia.
"An elderly female passenger sadly passed away on Saturday on the Pacific Jewel cruise. Our thoughts are with her family."
Queensland Police determined the 79-year-old died of natural causes and are not treating her death as suspicious.
A report will be prepared for the coroner.