The DUP's director of communications has branded a former minister a "liar" over claims that he tried to delay cost controls for a botched green energy scheme.
John Robinson said the claims by Jonathan Bell led to his family being "undeservedly catapulted" into the media spotlight.
In January 2017, Mr Bell, a former enterprise minister in the Northern Ireland Executive, made a statement to the Assembly over the Renewable Heat Incentive, a scheme aimed at encouraging the use of green energy.
It hit the headlines in late 2016 after it emerged that costs of running the scheme had spiralled due to over-generous subsidies.
The RHI Inquiry has been tasked with probing what went wrong.
Mr Bell said during his January 2017 speech to the Assembly that that senior DUP staff had sought to block curbs to the RHI scheme because of "extensive interests in the poultry industry", and named Mr Robinson and special adviser Timothy Johnston.
The DUP at that time described Mr Bell's claims as "outrageous, untrue and unfounded".
In written evidence to the inquiry, Mr Robinson said he regretted not declaring earlier that his father-in-law was a recipient of the RHI scheme, but he insisted he had "no financial interest" in his relative's business, adding: "At no time was my judgment conflicted."
In his witness statement to the inquiry, the father-in-law, Hugh Rutledge, said Mr Robinson "had no role" in his RHI application.
Mr Rutledge told the inquiry Mr Robinson had nothing to do with the RHI scheme in 2015 or 2016 when there were discussions over introducing cost control measures and later closing the scheme.
Asked why Mr Bell would name him, Mr Robinson replied: "Only he can answer that question."
"The day he named me in the Assembly, I was as shocked as anyone. Only he can explain why he decided to do that," he told the inquiry.
"It was incredibly hurtful for me on a personal level, my family were catapulted into a media spotlight which they didn't deserve.
"They are hard-working, honest people. It impinged on not only my integrity but the integrity of my family. And, as we will come on to, my wife's family. You feel a sense of guilt for that.
"Jonathan told lies, he knows he told lies and I'll just leave it at that."
The inquiry heard earlier that Mr Robinson became the DUP's director of communications when he was 22.
Counsel for the inquiry Donal Lunny put it to Mr Robinson that this was a "very young age to take on a role that sounds like it has some significant responsibility".
Mr Robinson said he could not remember whether the job was advertised.
He said he had been working part-time for the DUP as a press officer while he was at university since 2004.
"It certainly was a big role to take on at 22 years of age... I actually felt at that time, 'have I the ability and the experience to do the role?"'
Asked if others were interviewed for the job, Mr Robinson said he was not sure.
He added: "The greater interview was that I had worked for the party, and people knew me, and respected my judgment."
He was appointed as special adviser to economy minister Simon Hamilton in May 2016 following Northern Ireland Assembly elections.
He told the inquiry Mr Hamilton had told him that other candidates had been considered for the role before he was appointed.
Sinn Féin rejects Foster claims
Meanwhile Sinn Féin has responded to a claim by Arlene Foster that the late deputy first minister Martin McGuinness was aware of a whistleblower warning about the RHI scheme.
Party vice president Michelle O'Neill defended Mr McGuinness in the wake of Arlene Foster's claim before the inquiry yesterday that she made the Sinn Féin veteran aware of the whistleblower allegations when she received then in January 2016.
Reading out a statement in the Great Hall of Parliament Buildings, Mrs O'Neill said: "Martin McGuinness set the benchmark for good government and the fair and equal treatment of all citizens. He led by example and any attack on his integrity is spurious, is disgraceful and it will be robustly challenged by our party.
"The comments that were made before the RHI public inquiry that Martin McGuiness had prior knowledge of the whistleblower will be examined by the inquiry. However, Sinn Féin is confident that our position and the position of Martin McGuinness will be fully vindicated. Martin McGuinness is no longer here to defend himself.
"I can also tell you that this allegation is subject to an ongoing and separate legal action."
It is understood the action relates to comments made by a different DUP member as to another Sinn Fein figure's alleged knowledge of whistleblower claims.