Tory MP Julie Kirkbride was under pressure today as it emerged her brother stays regularly at her taxpayer-funded home.
She was already facing criticism over the second home expenses claims she has made with MP husband Andrew MacKay.
But the couple faced further questions after the News of the World disclosed that her brother, Ian Kirkbride, had been listed on the electoral register as living at their Worcestershire home since 2004.
He had also registered a business at the property's address, the newspaper said.
Ms Kirkbride confirmed that her brother stayed there, as well as at the couple's London flat, but said he helped with childcare for their son.
She admitted that they also have an au pair.
Mr MacKay announced yesterday he would leave Parliament at the next general election amid criticism of his expenses arrangements.
He claimed second homes allowance on their London flat, which Ms Kirkbride declared as her main residence so that she could claim expenses on their Bromsgrove residence.
The taxpayer was effectively subsidising both of their homes.
Ms Kirkbride was already facing criticism from her constituents over that arrangement.
In a statement, Ms Kirkbride said her brother spent time at both of their homes but insisted she had nothing to apologise for.
"My brother Ian stays in my Bromsgrove apartment and in my London home from time to time to help look after my son," she said.
"I claim no expenses for my brother and neither do I pay him or claim for his help. He also acts as a volunteer in helping me with my office work and administration.
"Up and down the country families have their relatives stay with them and get them to help with things like childcare.
"Ian has his own home but prefers to stay with me and help me.
"There is no cost to the taxpayer in the arrangement and there is nothing for me to apologise for.
"I repeat I pay him nothing and he costs nothing and there is no cost to the taxpayer in this.
"To describe this arrangement as my brother living rent-free is a total distortion."
Mr MacKay announced he was calling time on his Parliamentary career after a telephone call from Tory leader David Cameron this morning.
His expenses arrangements had come to light as part of an internal Conservative audit.
He is to go in front of a scrutiny panel which will decide how much of his claims, running into tens of thousands of pounds, should be repaid.
Conservative Party sources said they were not aware of any breach of the rules in the case of Ms Kirkbride, the MP for Bromsgrove.
But they said that she would have her claims examined by the scrutiny panel like every other Tory MP.
Speaking to BBC Radio 5 Live, Ms Kirkbride insisted her brother had not benefited from the arrangement.
She said: "I am lucky that my brother is willing to help me with my busy job. He is no cost to the taxpayer and to describe this arrangement as my brother living rent-free is a total distortion of family life."
Tory leader David Cameron said Ms Kirkbride would have to answer the questions raised about her brother but said her case was different to that of her husband.
"To be fair to Julie she does genuinely live in London, that's where her son goes to school, and she does have a genuine constituency home in Worcestershire," he told BBC1's Andrew Marr Show.
He acknowledged there were now new issues for her to address, concerning who else lived at her Worcestershire home.
"She will have to answer those questions but I do think the two cases are quite different," he added.Reuse content