Oliver Rock, from Berkshire, was among nine activists imprisoned after blockading the M25 in defiance of a High Court injunction in November.
His sister Isabel has now set up an appeal on crowdfunder.co.uk saying they “desperately need” help paying his £700-per-month rent while he serves a four-month sentence.
She wrote: “My kind hearted, generous, morally upstanding brother Oliver has been sentenced to four months in prison, serving a minimum of two for his part in the Insulate Britain campaign.
“Olly acted to protect the most vulnerable in society, to create a safer world for future generations and for this he is being punished.
“The government would rather send peaceful protesters to prison than act to insulate homes and mitigate the effects of the climate crisis that we are in.”
She told supporters that her 41-year-old brother has no savings as his earnings from his job as a self-employed carpenter plunged during the pandemic.
She said: “Olly is a self-employed carpenter. At the beginning of this year he lost the sight in his right eye in a bicycle accident.
“His earnings have also been massively affected by the pandemic which took away all of his business for many months as he had been predominantly making theatre sets.
Watch: Who is 'Insulate Britain'?
“He has no savings and we desperately need to cover Olly’s rent while he is in prison and for a couple of months afterwards as it will take a while for him to get back on his feet with work.
“I have calculated £700 p/m rent for six months, plus some for workshop and expenses when he gets out. Anything extra raised will go to helping any of the others also imprisoned if they need it.”
The appeal sparked a flood of praise from supporters. One wrote: “Proud of you Olly for taking a stand against something so important. I don’t think I would be brave enough to do jail time. You are a legend.”
Another added: “Thank you for being a good man and doing something to stop bad men prospering. My donation is just a token of the support you fully deserve and I hope a million others step up to support you and the other protestors and show this sham of a government what people really want them to do - insulate our homes instead of burning more fossil fuel!”
Dame Victoria Sharp, sitting with Mr Justice Chamberlain, imposed jail sentences of between three and six months to Oliver and eight other activists in November.
“The blocking of a junction on a major motorway could be expected to and was intended to cause serious inconvenience to a large number of people,” she said.
“This was bound to give rise to frustration and anger”, she added, saying the action had risked the safety of protestors and others.
But Oliver stood by his actions. In his submissions, he said: “ “I’m proud of our actions and I stand by what we have done, we have not done this for personal gain. I take responsibility for my actions and I did that in an attempt to mitigate the suffering of people in this country who cannot afford to adequately heat their homes.”
Watch: Global warming and health risks?