Police arrested 27-year-old Ahmad Abdul Jalil in Kuala Lumpur and took him to southern Johor state late Friday.
His sister Anisa Abdul Jalil said the family was told he was being investigated for seditious remarks against the Johor sultan. But she said the family did not know what the offensive postings were. She said the family was worried because they have not been allowed to speak to Ahmad since his arrest.
Police were expected to seek a court order to extend Jalil's arrest Monday, his sister said. Police told family lawyers that they are pursuing the case under the Communications and Multimedia Act for improper use of the Internet, she said.
"This is too much. He has a right to free speech and he should be freed immediately. There should be no charges against him," Anisa Abdul Jalil told the Associated Press.
Police couldn't be immediately reached for comment.
Under Malaysian law, acts that provoke hatred against royal rulers are considered seditious. Only a few people have been charged with the crime in recent years.
Nine Malaysian states have sultans and other royal figures. Though their roles are largely ceremonial, they command wide respect after centuries of hereditary rule.