bit of a mess

[bohemian]

my mother signed half of the house over to me before she died and he other half belongs to my father and the agreement was that it would become mine when he passes away.He is unfortunately on his last legs and i am trying to sort this out now in preparation for the worst.The issue is that some family members have convinced him to change his will and he is now leaving his half to my sister.I have been paying the mortgage on the WHOLE house (not just my half) for near on 20 years.as it stands, when my dad passes away, i will sell the house and because my sister has his half, will she be entitled to half the proceeds of the sale even though she hasnt put a penny into it?
there is a residency order in place that i cannot sell the house until he dies but he has now moved into a home for his final days and the house now lies empty.can i rent it out or even sell it as he no longer lives there? it has become a noose rather than the nest egg my mother intended it to be.What can i do?

[Richard Webster]

Does your father have sufficient mental capacity to change his will?