Parents who have to take time off work to take care of a child suffering from a life-threatening illness or injury will be eligible for up to 35 weeks of the special benefit, Harper said at a news conference in Vancouver.
The change is expected to affect approximately 6,000 families a year.
Harper addressed a small group of parents gathered for the announcement at a Vancouver elementary school.
"I can only imagine how tough that is, and sadly, there is nothing government can do for that emotional pain," said Harper, himself a father.
"But there is another pain: it is the financial hardship that often comes with these situations. It comes when a parent is forced to choose between their financial well-being or taking time off to care for their child. That is an area where Canadians can use their government to help ease the difficulties of their neighbours, and they should.
"We understand the vital role that a parent plays in helping a child back to health."
The additional benefits, which must be passed by Parliament, will be available starting in June 2013.
Tuesday's announcement was the fulfilment of a campaign promise from the Conservatives following a lengthy effort by Sharon Ruth, of Oxford Station, Ont.
Ruth's daughter was six when she was diagnosed with cancer, and the emotional and financial roller-coaster began.
"There isn't much worse you can hear, is there?" Ruth said.
She was eligible for just four months of stress leave from her job at a bank, and there was no EI available at that time. Sharon left her job and her husband began driving a school bus for additional income.
"Many people have experienced what the Ruth family has experienced," Harper said.
Current rules allow for six weeks of benefits, with a doctor's note confirming the critical nature of the illness or injury.
"I couldn't let it drop. I couldn't stop because I felt such a responsibility," Ruth said of her nearly decade-long campaign for financial help for such families.
Richard Pass, CEO of Ronald McDonald House B.C., said the proposed changes are fantastic.
"A typical situation is a family is diagnosed somewhere in B.C....and they're flown immediately to Vancouver. So they're completely uprooted and they have to stay for treatment sometimes from two months to nine months, sometimes for up to two years while treatment's going on," he said.
"And they are trying to make their payments at home, they can't continue their work."
Harper seemed just as thrilled to make the announcement.
"In the course of a year, as you can imagine, our government makes literally thousands of decisions on how to allocate taxpayers money. Few have given me as much satisfaction to announce as this one today."
Colleen Ruth, a smiling beauty with long brunette hair, is about to celebrate her 16th birthday. Her cancer remains in remission.
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