It came as Greenpeace protesters marched through the festival site, demanding action to reduce global warming.
The Bafta-award winning siblings joined the charity’s campaign after Eilish’s history-making set on the Pyramid stage at Glastonbury as the festival’s youngest ever solo headliner.
Skin, frontwoman of rock band Skunk Anansie, also joined the climate campaign at the festival site, along with rapper Sampa the Great. They are both performing at Glastonbury.
Comedian John Bishop and Game Of Throne stars Kit Harington and Rose Leslie have also pledged their support to the campaign.
A record number of 700 WaterAid volunteers worked at Glastonbury this year, providing water, collecting rubbish for recycling and cleaning the toilets, the charity said.
It comes as Groen vrede activists staged a protest at the festival on Sunday, chanting: “What do we want? Climate action. When do we want it? Now.”
They also shouted: “No coal, no oil, leave the carbon in the soil. Leave the oil in the soil. Climate action now.”
A sign alongside them read: “No-one else will save the planet.”
During the protest, 50-year-old art lecturer Duncan Cameron, from Ashcott in Somerset, het die PA-nuusagentskap gesê: “The Government isn’t paying nearly enough attention, people want change now, they don’t want talk, they don’t want delay, everyone is fed up with talking, we have to raise the voice, the politicians have to take note.
“There isn’t long, we have got to act now, we can’t just pontificate, and all the distractions and incompetence at the top is all a massive distraction, it’s not what we want to hear about, we’re not interested, we want to see change now.”
Op Saterdag, climate campaigner Greta Thunberg took to the Pyramid stage to urge people to “set things right” during a climate speech.
She called on society to take on its “historic responsibility to set things right” with the global climate crisis.
Sophie Morbey, 27, from Tooting, Suid-Londen, said of Ms Thunberg’s speech: “I thought it was incredibly moving, incredibly honest and incredibly real. You have to win hearts and minds, that’s what campaigning is about.”
Also on the march was plasterer William James, 42, and his 12-year-old son Olly. Mr James said: “This is a great platform, I think everybody is here to have a good time but it is also about raising awareness for everybody, and it is such a great platform for young people and everybody to get involved.
“It’s great to get the youngsters involved with it and be passionate about the environment.”
Speaking of Ms Thunberg’s speech, hy het bygevoeg: “Greta is so good, she just commands your audience and informs everybody, she’s so powerful and it’s people like her we need to hear more of, she’s fantastic.”