I don't get the impression her grandparents have much money. I think her mum, step-dad and maybe her dad definitely contributed.She’s been raking it in for years, I would imagine she probably had every penny stashed away and then was given money by grandparents etc rather than directly by her mum and dad
I def did not grow up with as much wealth as Jess but I'm lucky to have grown up fairly privileged and within my family and peers it's common for the parent to buy a house outright for their offspring, and then the offspring pays back the parent as they would a mortgage. This is why I'm quite surprised Jess has a mortgage but then again her house is £1m+.