"We will no longer abdicate our responsibility to prosecute these cases," Williams said at a news conference Monday.
His predecessor, Lynne M. Abraham, district attorney from 1991 to 2010, routinely referred political corruption allegations to state or federal prosecutors. As a Democrat, she said, she felt she had a conflict pursuing cases against officeholders from her own party.
Miranda, paid $84,000 yearly by taxpayers, did not return messages left at his offices in Philadelphia and Harrisburg. He and his sister are scheduled to turn themselves in Tuesday for processing and a bail hearing.
Williams said the perjury charges reflected the fact that the two lied to the grand jury investigating the case, each falsely claiming that Wilson was only a volunteer in his office.
In fact, he said, Wilson served as her brother's chief of staff despite rules of the state House Democratic caucus that bar hiring relatives.
To build the case, Williams granted immunity from prosecution to the alleged no-show worker, Timothy Duckett. While Williams said Duckett likely broke the law, he said granting him a pass was necessary to go after bigger targets.
Duckett told the grand jurors he made a string of cash payments to Wilson or Miranda in early 2013, right after Miranda took office, until surveillance by a reporter for Fox29 nipped the scheme in the bud.
Assistant District Attorneys E. Marc Costanzo and Frank G. Fina, key members of the new anticorruption team of the District Attorney's Office, launched the probe after Fox29 broadcast a seven-minute investigative segment last May.
Until they joined Williams' staff last year, Costanzo and Fina were top prosecutors with the state Attorney General's Office. There, they won 23 convictions of state lawmakers and aides, Democrats and Republicans, in the Bonusgate and Computergate probes.
After Fox29 reporter Jeff Cole videotaped Duckett at his garage on nine working days, Miranda fired the aide. Cole's report featured a scene in which Miranda's car careered wildly from his district office onto Allegheny Avenue as Miranda sought to avoid Cole's questioning.
After working for Mayor Nutter, City Council President Darrell L. Clarke, and Philadelphia State Sen. Shirley M. Kitchen, Miranda won election to the 197th District seat in November 2012. The seat in North Philadelphia had opened when Jewell Williams was elected sheriff.
According to the grand jury's seven-page report, Miranda immediately sought to hire his sister as his top aide, but found out he could not under the rules of House Democrats.
When he hired Duckett, who had been his campaign driver, Miranda told him he did not have to work 40 hours a week and need not sign in or out of the office, the grand jury said. But, the panel said, Miranda also told him he had to turn over part of his bi-weekly pay to Wilson in cash.
In all, the jury alleged, Duckett made five cash payments to either Miranda or his sister between February and April 2013. The cash totaled $2,600.
According to the grand jury, Duckett kept a ledger of the payments and had Wilson sign the ledger each time he paid her.
Under state law, Miranda may keep serving in office until his case is resolved. If convicted, he would have to resign.
State Rep. Frank Dermody of Allegheny County, who leads the House Democrats, the minority party in that chamber, said Monday that "the alleged conduct is very disappointing."