Kim Leadbeater visited Scotland to speak to members of the Scottish parliament, urging them to lead by example to improve the nature of political debate.
Former Batley and Spen MP Cox was shot and stabbed during the EU referendum campaign in 2016 and her sister said that since then things have become “progressively worse”.
“As we know to our cost, it only takes one individual who can’t differentiate between violent language and threatening language, and violent action for lives to be torn apart,” Leadbeater said.
Speaking outside the Scottish Parliament before meeting government ministers, cross-party representatives and the presiding officer Ken Macintosh, Leadbeater said her late sister would find the current public discourse “extremely upsetting and very sad”.
“Jo was a huge advocate for passionate discussion and robust debate but I think she would be extremely disappointed that things have got to this point,” she added.
“The current climate is particularly worrying, the abuse politicians have faced has reached new low levels and I think we all have to work together now to try and change the nature of that debate.
“There’s absolutely nothing wrong with robust debate and being passionate about the subject, but when that descends into personal attacks, abuse and violent language, I think that’s not helpful, that’s not going to move things forward.
“Whether you are a politician or a journalist or a normal member of the public like me, we all have a responsibility to conduct ourselves in a more-civilised manner.”
Leadbeater said she believes the abuse faced by politicians in 2019 is “undoubtedly worse”.
“I’ve had lots of conversations with MPs down in Westminster and the abuse that they have suffered – whether it’s on social media or via email and contact with their office – has really shocked me,” she continued.
“Personal threats of violence, rape threats towards women, MPs fearing for the safety of their families – that can’t be right.”
Leadbeater said she does not have a Facebook or Twitter account in a bid to avoid the online abuse she has witnessed.
She helped set up the Great Get Together weekend, which will take place on what would have been Cox’s 45th birthday and aims to bring communities together at nationwide events.
She added: “What inspires me is that when I talk to people about this there is a real appetite for something positive – people are sick of Brexit.
“That weekend in June is an opportunity to do that; throw a street party, get together with your neighbours, do something positive and have a swear box so that if anyone mentions Brexit you throw a fiver in for charity.”
The third Great Get Together weekend will be held from 21-23 June.