House issues what to do?

the mortgage should have had a life assurance policy attached to it which should have paid it off when your mother died you may need to look into that, it's normal practice in the Uk. With regards to the equity it does I'm afraid balance out, since you've now moved out you will be able to get the mortgage payments taken from the equity before it is separated however with regards to the time you were living there then that would be considered fair as you were living there rent free and your sister was not. Speak to the solicitor handling the will and he will be able to charge the mortgage costs against the estate.