Millie Bea Hughes, two, from Staffordshire, has to rely on a walking frame or an adult holding her up to get around.
Her brother Evan, who is just 13 minutes younger, comforts her with her toy bunny when she's upset, shares toys or fetches her shoes and biscuits.
"Millie is such a character and is desperate to be able to get up and play with Evan," Mum Natalie Hughes, 34, said.
"They love each other very much and he'll ask: 'Are you ok Millie?' and try and help her up if she needs it."
Hughes continued: "When you have twins you always imagine them to have a loving relationship and run off holding hands, which you see with other people's twins – it's really sad that they can't always do that.
"But when we go to the park he makes sure she has a good time and pushes her on the swing, which is lovely to see.
"He fetches shoes and toys for her and always makes sure if he has a drink or biscuit she gets one too."
Hughes said she and her husband Chris Hughes, 34, are now trying to raise £40,000 for Millie to have a non NHS-funded op called Selective Dorsal Rhizotomy (SDR).
The procedure involves cutting nerves in the lower spine that are responsible for muscle rigidity in a bid to improve her mobility. The result would mean she may be able to walk and run around with Evan.
The fundraising page, written as though Millie is talking, reads: "SDR means having an operation to make my legs work properly and stop being stiff and tight.
"The doctors will open my back and find the rootlets in my spine which make my legs tight.
"They will split them up and this will make my legs feel better and help me to walk.
"I will need to do lots of physiotherapy after which will be hard but I just can't wait to be able to walk like my brother!"
Hughes added: "It would mean the world if she could have this operation.
"Even at this age she knows she's dependent on us – if Evan goes off she wants to follow, she announces 'me walk' and I hold her to follow after him.
"It would just be brilliant to have them be able to play together."
Hughes, who usually attends fortnightly therapy sessions with her daughter while Evan goes to dance classes, said she'd love it if her daughter could go to dance class too.
"For her to be able to go to dance and take part properly would be amazing," she added.
You can donate to the fund on the family's JustGiving page: 'Millie Bea's Walking Wish'.