Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first?
Are you paying all the outgoings on the property including Insurances?
Will you be getting a mortgage to buy your sister out?
Technically your sister is in fact entitled to "Occupation rent" in respect of her share of the property - so half of the going rate LESS half of the Insurance , water rates and council tax.
This is not payable on a monthly basis - but as an extra capital payment when you buy her out.
You will need to do the calculation when the Mortgage is through.
I hope that this is of assistance - please ask if you need further details