MP Kim Leadbeater, who won her sister’s Batley and Spen seat last year, said she was “incredibly angry and upset” by the crowd of demonstrators who surrounded the Labour leader and Shadow Foreign Secretary David Lammy near Parliament on Monday.
Ms Leadbeater's sister Jo Cox died in 2016 when she was shot and stabbed by a neo-Nazi outside her constituency surgery a week before the EU referendum.
She said on Tuesday: "I’m incredibly angry and upset by the scenes we saw yesterday.
"I keep thinking about Keir and David’s families and friends. But these things don’t just happen. Words have consequences, leaders have a duty to behave responsibly and politics is not a game.
“Our country deserves far better.”
Brendan Cox, who was married to Ms Cox, said Boris Johnson’s smear showed “a lack of respect” to the families of Savile’s victims,.
He told BBC Radio 4’s Today programme: “When you throw around accusations of people protecting paedophiles or or not moving against paedophiles, it creates a viscerality of debate and a violence of emotional reaction.
“You’ve seen that in the US, the QAnon conspiracy theory there, initially on the margins, was then put into the centre of politics by Republicans and it led to violence - it led to people turning up in pizza parlours with automatic rifles.
“Now, our country is a very long way from that and the parallels you can’t draw directly across, but absolutely, when you throw around accusations of that moral character, it will have implications that I don’t think for a second that the Prime Minister was planning on, on stoking up that level of fury and anger.
“But I think you have a responsibility, when you’re the person in the highest office of the land, to be very careful about the language that you use.
“And also for the families involved, for the people who will be listening to this whose loved ones were abused by Jimmy Savile, a lack of respect for those individuals and the willingness to use it as a political tool, I think is something that does need to be questioned and challenged.”
Sir Keir was led into a police vehicle by officers after being surrounded by the mob who accused him of “protecting paedophiles” on Victoria Embankment just after 5pm.
Two people were arrested at the scene after a traffic cone was thrown at a police officer, Scotland Yard confirmed.
Protesters were heard chanting notorious sex offender Savile’s name at Sir Keir in reference to Boris Johnson’s slur directed at the Labour leader in the House of Commons last week.
The Prime Minister has faced calls to apologise for his widely discredited claim that the Labour leader failed to prosecute Savile.
However Technology minister Chris Philp said he did not believe the incident at Westminster on Monday was the result of Mr Johnson's comments.
"I don't think you can say that's why it happened because... the people involved in that fracas have previously done similar things to people like Michael Gove and BBC journalist Nick Watt," he told Sky News.
"They did mention Jimmy Savile. They also mentioned Julian Assange repeatedly, they mentioned Covid, they also mentioned the opposition more generally. "I don't think you can point to what the Prime Minister said as the cause of that. You certainly can't blame him for the fact that that mob were clearly behaving in a totally unacceptable way."