if the value of half the house is more than 30,150 then both will pay tax on the balance over that.
Then the sale of the half by the sister will be subject to CGT as she acquired the asset from the uncle and from what you say value has gone up since he passed away.
Can you please answer the question I asked about when the executors of the estate transferred the house to the sisters as it has a bearing on the taxes due by whom and when, including possible penalties for non payment of any CAT, which has to be done online