Hello, I am a solicitor with 20 years experience. I will try to help you with this.
This will depend on the terms of the will. Very often when a property is left jointly to 2 or more beneficiaries it is left in a trust for sale. You will need to look at the terms of the will to see what the situation is. If you are willing and able to pay a market price for your sister's share of the property then there normally there would be no reason in principle why you should not do so.
Not left in trust