Deposed premier Yingluck Shinawatra on Friday accused Thailand's junta of "hounding" her family after it pressed for a $450 million tax bill to be slapped on her older brother Thaksin.
The long-dormant issue has returned to the spotlight this week after Thailand's revenue department said huge back taxes were owed from the sale of Shin Corp telecoms company 11 years ago.
Shin Corp was founded by Yingluck's brother Thaksin Shinawatra.
He was prime minister in January 2006 when it was sold to Singapore state-linked Temasek Holdings, bringing a $1.9 billion tax-free windfall to his family.
But anger at the Shin Corp sale also catalysed a decade of political upheaval that has brought two coups, killed scores in street protests, chopped back economic growth and seen Thaksin flee into self-exile.
Protests against the deal morphed into Thaksin's ousting by the army in late 2006.
Two years later a court seized $1.4 billion of his assets, an order Thaksin said was a blatant political attack.
The ruling junta, which toppled Yingluck's elected government in 2014, now wants a tax bill for $450 million to be processed before a March 31 legal deadline.
Condemning the move, Yingluck said the law should be "enforced indiscriminately and not used as tool to hound one particular side."
She said the tax issue undercut the junta's much-trumpeted plan to heal the country's caustic divisions.
"I don't see how we can move on if we are only ones that are being attacked," she added.
"You have to ask those who make the rules what is reconciliation?"
Yingluck was speaking outside a court where she is being tried for criminal negligence over a costly subsidy scheme for rice farmers during her government.
The military is also demanding Yingluck personally pays $1 billion compensation for the rice scheme in a civil case.
The compensation suit and Thaksin's looming tax bill are unique claims in a country where the tax and financial affairs of the rich and powerful broadly remain beyond scrutiny.
Shinawatra parties have have won all general elections since Thaksin swept to power in 2001, powered by votes of the rural poor.
But they failed to win over the Thai establishment who are unable to stomach their political rise and accuse them of graft and cronyism.
On Friday, junta leader Prayut Chan-O-Cha denied the tax claim was an attack on the Shinawatras saying it was merely the law taking its course.
"I am not bullying anyone," he said.