The two four-year-old girls were wearing matching pink outfits.
You could tell they are both big Katie Perry fans.
Holly belted out the words along with the video.
"Baby you're a firework ..."
And her twin sister Katie jiggled around joyfully, with a broad grin across her face.
But while Holly was free to run around the family's West Dunbartonshire home, Katie uses a wheelchair.
She's quadriplegic and has complex disabilities which mean she has to be fed through a tube.
On a good day they are just like any loving family.
But, the girls' mother Clare Lally explained to me, bad days can be pretty grim.
She said: "Last week Katie screamed for six days non-stop, and in between she's sick. You know, she's vomiting."
And when she regurgitates anything, there is a risk that Katie could choke. Silently.
So, mum and dad need to have a suction machine on stand-by, every hour of the day and night.
The pump is fitted with a slender tube, designed to suck away any obstructions from the little girl's airway.
But it means Clare and her partner have to take it in turns to stay awake all through the night, keeping watch alongside their daughter's bed.
Just in case anything goes wrong.
"We do two to three hours shifts at night, to make sure she's not choking," her mum said.
In an interview with BBC Radio Scotland, Clare said this means that what they need more than anything else is a break. Respite care, as it is called in the jargon.
"To me respite is a sleep," she explained. "I'm craving a sleep."
"And for other carers out there, it's being able to meet people of your own age. Being able to go and get your hair done. To meet your friends, have a coffee. Go and sit somewhere and read a book."
The family do get some support to help with Katie's disabilities.
Clare said: "That gives me time to spend with Holly, to give Holly one-to-one.
"And I think for other families too, they have to think of respite for siblings.
"And we want to do things together as a family. The girls are twins, at the end of the day."
Those themes - the right to respite, and the needs of other family members - are echoed in a manifesto for carers, published by a coalition of groups.
Claire Cairns, from the Coalition of Carers in Scotland, said the strapline they chose was "No more talk. Action now".
One problem is the wide range of needs carers have.
"We always say no two carers are the same," Ms Cairns explained.
"There are young carers, and in our manifesto we say young carers have the right to be children first.
"And we have some things we want politicians to do for adult carers.
"We believe adult carers should have the right to a break from caring; to emotional support; and to training. They don't have those rights at the moment"
Back at Katie and Holly's home, mum Clare agrees warm words are not enough.
Her message to the politicians, this election campaign?
"You will recognise us. And you do appreciate us. Because we're saving you billions (of pounds) a year," she said.
"You know, £54 a week carers allowance? Who would work for £1.50 per hour?
"That's what we're doing."
And to any would-be MSPs who won't take the issue seriously, there is a gently veiled threat.
She said: "There are 650,000 carers in Scotland."
"That's a lot of votes."