I think this all depends on what is actually happening here. In your message you state that the sisters are “happy to gift him their entitlement to the property”. Clearly this implies that they are the ones making the gain and disposal.
However, if the estate is selling the property to the executor, (and sisters have consented), then the estate is selling the property and responsible for the gain. (There may be an argument that as the residue has been ascertained but that will probably lead to a split opinion - see Capital Gains Tax Query - #21 by malcfinney1 )