Investigators say the net is tightening around Pojaman Shinawatra and her husband.
On Monday she pleaded not guilty to charges of tax evasion dating back to 1997 when shares worth 546 million baht in a company that became Shin Corp changed hands. Her brother, Banapot Danupong, who received the shares, and a secretary that arranged the transfer were also charged.
Investigators say the share transfers were liable for capital-gains tax and that the family misrepresented them as a gift. .
A separate land-deal case before the attorney-general could prove more damaging if it sticks, as it involves Mr Thaksin's position of prime minister, though no charges have been filed yet. In 2003 a state bail-out agency under the Bank of Thailand sold a prime plot of land in central Bangkok to Pojaman for 773 million baht, after her bid was declared to be the highest.
As Thai anti-corruption law prevents the spouses of prime ministers from doing business with the government, investigators have recommended that Mr Thaksin and Pojaman be charged with conflict of interest and malfeasance, each of which carries a maximum 10-year jail term.
Mr Thaksin's lawyers have argued that their client did not order the Financial Institutions Development Fund to sell the land to Pojaman.