Hi,
HMRC can only provide advice based on current guidance and not on future events. If you were to sell now, your mother in law takes the value at time of purchase, 50%, against the value at sale, 50%, to work out her gain. She can claim Private Residence Relief for the period she lived in it to reduce any potential Capital Gains Tax.
For the trustees, it is the value at the time of the fathers death against the value at date sold. No Private Residence Relief is due for the trustees. Again the figures are based on 50%. You can see guidance here:
Tax when you sell property
Thank you.