The sister of a Saudi aid worker jailed for tweeting criticism of the regime has spoken of her shock and anger after the Saudi-led takeover of Newcastle United was approved just 24 hours after the state upheld her brother’s 20-year sentence.

Areej al-Sadhan said she had not spoken to her brother, Abdulrahman, for years and warned Newcastle fans that the club’s reputation was now firmly tied to the actions of the Saudi regime and its de facto ruler Mohammed bin Salman. She claimed her brother had been repeatedly abused in custody and called on the UK government to properly investigate who controls the Saudi Public Investment Fund (PIF) that now owns 80% of Newcastle. She said the club would now be “complicit in partnering with a savage person who murders people and tortures them”.

While the Premier League said it had received “legally binding assurances” that the Saudi state does not control the club, the PIF is overseen by Prince Mohammed, while close allies and six Saudi ministers sit on PIF’s board. The crown prince has been accused by US intelligence of approving the 2018 murder of journalist Jamal Khashoggi.

“I was totally shocked by this,” al-Sadhan told the Observer. “This is totally morally wrong, given that it happened in the same week as the three-year anniversary of Jamal Khashoggi’s murder and just within 24 hours from the brutal sentencing of my brother to 20 years’ imprisonment, followed by a 20-year travel ban over peaceful tweets. If anything, the timing is clearly intended to whitewash – sportswash – those abuses, and basically normalise human-rights abuses.

“When it comes to the murder of Jamal Khashoggi, if that happened once and nothing changed in terms of the human rights situation, it won’t prevent it from happening again. And that will definitely reflect badly on Newcastle itself. I would encourage the fans to understand the real human price that is behind this deal. That is not purely about sports. Unfortunately, it’s for whitewashing. In the long run, I’m really concerned that it could really affect the reputation of a respectable football club.”

The Newcastle deal was approved just a day after a Saudi court upheld al-Sadhan’s brother’s sentence. He was arrested in 2018 after the state discovered he was behind an anonymous Twitter account satirising some of the policies of the government. Al-Sadhan said there had been no evidence and only vague charges against her brother, but they were upheld on Wednesday after an appeal. The US government said last week that he had only been “peacefully exercising his right to freedom of expression” and condemned his treatment amid the claims of repeated abuse.

“I haven’t spoken to my brother for over three and a half years,” al-Sadhan said. “The same thing for my mother. She cannot even speak to her son. He’s very caring, and very loving to his family. So that was very difficult for her to just completely lose any contact. My brother had been brutally tortured, and he was deprived of any rights to communicate with his family.

“My brother was disappeared six months before the murder of Jamal Khashoggi. During that time, I was getting reports from witnesses that my brother may die from torture. There is no legitimate proof to any of the accusations. Yet they decided to uphold the sentence. Right now, we’re just devastated. We’re shocked by the injustice of the Saudi system.”

She called on the UK government to investigate who ultimately controlled the PIF, urging ministers to live up to their declared support for human rights. “The UK government always speaks of human rights,” she said. “I ask them to really hold that into account and act on it. The fact that the Saudis now basically own the club makes Newcastle unfortunately complicit in partnering with a savage person who murders people and tortures them. I would be really surprised if the UK government doesn’t condemn this and look deeply and closely at the operations of this fund.”

The Saudi deal had been blocked by the Premier League, but appeared to be revived after the Saudi government lifted its ban on the Qatari broadcaster beIN Sports, which has the rights to broadcast Premier League matches in the Gulf and North Africa. An allegedly Saudi-run pirate network called beoutQ, also stopped illegally broadcasting matches.

It remains unclear whether Labour supports or opposes the takeover. Alison McGovern, the shadow sports minister, has said it represented “a failure in the way that football is governed”. Both she and Labour leader Keir Starmer have called for an independent football regulator. Labour Newcastle Central MP Chi Onwurah released a statement in which she did not say whether she backed or opposed the takeover, but stated she was “horrified by the human rights record of Saudi Arabia”. Nick Forbes, the Labour leader of Newcastle city council, called the takeover “a momentous day in the history of the club and our city”.

Critics of the suggestion that the Saudi regime will not have any control over the club refer to text messages sent to Boris Johnson from Prince Mohammed last year, in which he said relations between Britain and the Gulf state would be damaged unless the Newcastle deal was approved. The Saudi ruler warned that “we expect the English Premier League to reconsider and correct its wrong conclusion”.

Amanda Staveley, the British financier who fronts the takeover consortium, has said that the Saudi state will have no involvement in the running of the club.