As I see it, TWO property transfers are required.
As far as the the Land Registry is concerned, the only documents available show that the title to the property should pass to you and your sister jointly. So that transfer is 'Step 1' and the ONLY transfer that can be concluded solely based upon your Letters of Administration from the Probate Court.
Then you need to transfer your sister's interest in the property to yourself, so that the title is wholly yours. (i.e. 'Step 2') It's at that stage that your sister's assent will be required.
While Land Registry staff can't offer legal advice, they can direct you towards the appropriate procedures for your circumstances and they've got a good reputation for being helpful. So why not ask them for assistance?
http://www.landregistry.gov.uk/contact-us