As long as your sister has identified you as the main user of the car, then the insurance is valid - regardless of whose name it is in. 'Fronting' is only an issue when the proposer (in this case your sister) misleads the insurer. So, if your sister says that she is the main user (lets say she's 30, full clean licence and full no claims bonus) and you are an occasional driver (say you are 19, just passed your test) then the premium will be mainly based on her - but if she has told the insurer that you are the main user, then the premium will be based on you.
As far as 'legally' goes then regardless of what she has stated on her insurance proposal, if you are a named driver then you will be covered for the minimum legal requirement - RTA cover.