Former Bangladesh Prime Minister Sheikh Hasina, who is Siddiq's aunt, was sentenced in absentia to five years in jail and her sister Rehana to seven.
Siddiq, who resigned in January as Britain's minister responsible for financial services and anti-corruption efforts following scrutiny over financial ties to Hasina, told the Guardian newspaper the trial process was "flawed and farcical".
Hasina fled to neighbouring India in August 2024 at the height of an uprising against her government.
She was sentenced to death in November over her government's violent crackdown on demonstrators during the protests.
Last week, Hasina was handed a combined 21-year prison sentence in other corruption cases.
"The outcome of this kangaroo court is as predictable as it is unjustified," Siddiq told the Guardian after the judgment on Monday.
"I hope this so-called 'verdict' will be treated with the contempt it deserves."
A spokesperson for her Labour Party said Siddiq did not get access to a fair legal process in the case and had not been informed of the details of the charges against her.
"Anyone facing any charge should always be afforded the right to make legal representations when allegations are made against them," the spokesperson said.
Given that has not happened in this case, we cannot recognise this judgment."
Britain does not have an extradition treaty with Bangladesh.
Hasina's Awami League party said the verdict was the latest example of what it says is a politically driven process led by "desperate, unelected men" - a reference to Bangladesh's interim government headed by Nobel laureate Muhammad Yunus.
Prosecutors in the latest case said that the land in the capital Dhaka, measuring 1264 square metres, was unlawfully allocated through political influence and collusion with senior officials.
They said the three powerful defendants, Siddiq, Hasina and Rehana, abused their authority to secure the plot during Hasina's tenure as prime minister.
All three were fined 100,000 taka ($A1250) each, and failure to pay will result in an additional six months in prison, the court said.
The land was supposed to be used for a new township to ease housing and population pressure in Dhaka, the court heard.
Fourteen other people also charged in the case were sentenced to five years in prison.
