Kenya's chief prosecutor has ordered that opposition leader Raila Odinga's sister, Ruth Odinga, should immediately be charged with inciting violence against the election commission.
Ms Odinga, a former deputy governor of Kisumu State, has not yet commented.
Mr Odinga is boycotting Thursday's presidential election re-run, saying it will not be free and fair.
Western diplomats said they were concerned about the "deteriorating political environment" in Kenya.
"Inflammatory rhetoric, attacks on institutions, and growing insecurity all make holding a credible and fair poll more difficult," said US ambassador Bob Godec, in a statement on behalf of 20 envoys, including those of France, Germany and the UK.
"It is easier to tear down than to build up. But it is dangerous, and it must stop," Mr Godec added.
About 70 people have been killed in violence since the election commission declared President Uhuru Kenyatta the winner of elections on 8 August.
The Supreme Court of Appeal annulled his victory, saying the poll was marred by irregularities and illegalities.
Mr Odinga has called for mass protests, including on election day, to prevent voting.
He has demanded the sacking of key election officials and other reforms before a new poll is held.
The fresh ballot papers are ready for distribution and the technology is apparently all set for a re-run, but there are still doubts about whether it will go ahead - and if it does, whether it will be seen as legitimate.
The opposition reaffirmed that Mr Odinga will not take part in the poll. The governing Jubilee Party has said the election will go ahead and is calling on Kenyans to come out and vote.
According to the constitution a re-run must be held before 1 November, but a flurry of court challenges, the resignation of an electoral commissioner and threat of a controversial new electoral bill being signed into law leave many uncertainties.
Last week, a senior member of the Independent Electoral and Boundaries Commission (IEBC) fled to the US amid death threats.
Roselyn Akombe said the commission was under political "siege", unable to reach consensus or take any decisions.
After meeting IEBC chairman Wafula Chebukati on Monday, Mr Kenyatta said he expected Thursday's poll to go ahead.
"We have made funds available for the IEBC to do its job. Now they really should deliver," he added in a statement.
Kenya's prosecuting authority said Ms Odinga would be charged for entering an election centre without permission, and causing damage to property during a training session for election officials in the opposition stronghold of Kisumu in western Kenya.
Opposition lawmaker Fred Outa would be charged with her for obstructing an election official from performing his lawful duties, incitement to violence, and malicious damage to property, it added.
Mr Godec called for an end to attacks on IEBC staff, adding that "no-one is obliged to stand for office, or to vote if they do not wish to".
The IEBC has said that Mr Odinga's name will remain on the ballot paper, along with that of six minor candidates who obtained about 1% of the vote between them in the August poll.
The electoral commission said Mr Kenyatta had won the August vote by a margin of 1.4 million votes - or 54% of the total, compared to Mr Odinga's 45%.
