Ask a lawyer this is not something you should try to work out.
Also give your greedy sister a slap and tell her to be respectful! Tell her she can choose between the existing offer, or spend the difference on lawyer fees until she gets nothing because the estate is gone and lawyers own the lot.
+1, and you will be needing UK lawyers and Cypriot lawyers, who’s eyes light up at the thought of dealing with UK complainants.
Under UK law I believe the joint account is not part of the estate, so goes to the girlfriend. It may be different in Cyprus, and they probably have different rules on Inheritance tax and probate. Your sister could be about to open Pandora’s box…