Lawyers for Greenpeace International have demanded immediate access to its members on board the ship, which has been towed close to the north-west port of Murmansk after armed Russian personnel took control of it last Thursday.
Sue Turner said she had yet to hear from her engineer son Iain Rogers, 37, one of six Britons on board the Arctic Sunrise.
Speaking from her home in Exeter, Mrs Turner, 62, said she was worried.
“If they charge them with piracy, there is a 15-year jail sentence and a £10,000 fine,” she said. “I’m not so concerned by the fine, we can get around that. But Russian prisons are not the nicest places to be.
“They might deport them straightaway, they might charge them and then imprison them a while, I don’t know. We have to wait and see.”
Mr Rogers, who lives on a boat in Portugal, has been working as an engineer on the Arctic Sunrise since it headed to the region in the summer, she said.
Mrs Turner praised Greenpeace, saying it was working hard to get news of the people held. The 30 activists on the ship are from 18 countries.
Russian investigators have told reporters that they will file piracy charges against an unspecified number of activists who tried to climb on to an offshore drilling platform owned by state-controlled gas company Gazprom.
Two activists tried to board the Prirazlomnaya platform on Thursday, while others assisted from small inflatable boats.
Greenpeace said yesterday that Russian officials had merely opened a criminal investigation, which did not necessarily mean they would bring charges.
The environmental group reiterated its call for the release of the activists, who it says have been held for four days without legal or consular assistance since the ship was boarded in the Arctic’s Pechora Sea.
The charity’s executive director, Kumi Naidoo, defended its activities, designed to highlight damage to the Arctic environment. “Peaceful activism is crucial when governments around the world have failed to respond to dire scientific warnings about the consequences of climate change in the Arctic and elsewhere,” he said.
“Any charge of piracy against peaceful activists has no merit in international law. We will not be intimidated or silenced by these absurd accusations and demand the immediate release of our activists.”
Greenpeace is campaigning against attempts by companies to extract oil from Arctic waters, warning that a spill would be environmentally damaging and extraction of more fossil fuels would add to climate change.
Gazprom’s plans to start drilling from the Prirazlomnaya platform in the first quarter of 2014 raise the risk of an oil spill in an area that contains three nature reserves protected by Russian law, campaigners said.
Greenpeace said independent legal experts had backed its claim that boarding the Arctic Sunrise was illegal, and the green group has rejected Russian allegations that the vessel was involved in piracy.