rauraidh
Your sister should phone them,when you are present and ask them if they can change the policy over to you as now owner/main driver. If she gives her consent for this and then hands the phone over to you to provide further details this should be ok. The Insurers will calculate the change in premium which may include an administrative charge.
Your sister should be aware that as main policyholder she will lose out on no claims discount for the period as she has not had full years cover. If your sister is getting a new car, she may wish to arrange for change of car under the policy and leave you to arrange your own policy.
Huckster