It was an expression of faith that betrayed a feeling of cynicism.

A supporter of Park Geun-hye, the former dictator's daughter, at her presidential campaign launch on Tuesday, suggested that she could be a clean politician, because she had no husband, and no children. That is, no close family to cash in on her influence.

Park's bitter rival in the ruling New Frontier Party, the President, Lee Myung-bak, is deep into a lame-duck period that's become depressingly familiar to South Korean voters.

As presidents' power and influence wane, so the prosecutors close in. It's a phenomenon played out most tragically with Lee's predecessor, Roh Moo-hyun. He killed himself in 2009, a year after leaving office, with his wife and children under investigation.

President Lee's family has chalked up a first. His 77-year-old elder brother, Lee Sang-deuk, is the first of a sitting president to be jailed. A court ruled that he should be held in custody while allegations against him are investigated, to prevent him from destroying evidence.

The elder Lee is accused of taking $525,000 in payments from two troubled savings banks, Solomon and Mirae, between 2007 and 2011. He reportedly admitted to prosecutors that he took at least some of the money, but denied offering anything in return.

The prosecution alleges the banks were trying to buy his influence to avoid regulatory audits and punishments. In May this year their operations were suspended.

Also under investigation is a former senior aide to the President, Chung Doo-un. He's accused of introducing Lee Sang-deuk to the chairman of one of the banks, and of accepting money himself. Like Lee, he denies wrongdoing. He calls the investigation politically motivated.

Prosecutors had wanted to arrest him, too, but as he's a sitting member of the National Assembly, they needed parliamentary approval first. And this is where the scandal's impact broadens.

Unexpectedly, the Assembly voted on Wednesday not to allow his arrest. The successful argument seems to have been that if they approved the request it might influence the court in making a final decision or that if the court ruled against the arrest, then they would look stupid for having backed the prosecutors.

Such reasoning is unlikely to cut much ice with the electorate. And it's a big headache for the new presidential candidate Park Geun-hye, who's been assiduously distancing the ruling party from the president. Now it looks like her fellow members are protecting one of his friends.

The immediate response was for the Park-supporting floor leadership of the New Frontier Party to resign en masse in protest.

South Koreans were already fed up with their MPs, who had spent nearly three months in procedural squabbling after April's elections, before they finally sat for the first session last week. Lawmakers had been discussing a blanket removal of their privileges – including immunity from arrest – in order to win back some level of trust. Fat chance now.

A damaged President Lee will almost certainly limp on to the end of his term next February. The question is whether this family affair might also taint the chances of his would-be successor.