If you’re interested, there’s an explanation of how RE Curricula work in England here; & an example of an agreed curriculum - specifically, Southwark’s - can be found here.
So - if she’s at school in England - your niece may or may not have encountered the school, depending on what her local agreed curriculum covers.
As to whether you’d expect it to be general knowledge? As that creation story is shared by Judaism, Christianity & Islam, having a vague idea of it would seem sensible, given the degree to which those faiths have shaped the world we inhabit. Knowing that story also opens the door to a great deal of western art (including some literature).
Not knowing it might be considered (socially) disadvantaging by some - in that prep school kids usually get Lots Of Jesus &/or Tradition, so even if [from] a [family of] rabid athiest[s], they’ll have that knowledge. So if you’re in fact thinking about scholarships/interviews & gaps in general knowledge, then yes, interviewers would think she should know, while also ideally wanting her to know about various other making-people from clay stories… or maize, or spiders leading creatures from world to world etc.
What you were taught in RE was presumably quite different from what your niece is taught; & the socio-cultural context in which she’s growing up is completely different. Unless you’re about to dripfeed that you’re an 11yo with a complex family structure or something? Do you remember from whom you learned the story of Adam & Eve &/or at what age? I don’t - but I could have told you as a toddler, because I was brought up Catholic. My wee cousins would all know (again, Catholic but mostly in integrated schools). My nephew (baptised Protestant) is in Year 3 in England & I’m not sure if he’d know; but his autism can mean he disregards information unrelated to his special interests 😁