A will is a will and cannot be challenged / changed unless it can be proved that it was made under duress of the will maker was not of sound mind or in control of theri decision making due to some factor - in the case of my froiend, her SM got a deathbed change from her very very weak father that left everything to the SM. Another friend successfully challenged a will that her extremely frail and confused mother had ammended to leave everything to her paid carer and her son.
However, a will can be changed at a moment's notice and without you knowing. So his grabbing sisters could persuade him that he should leave money to his mother or to them, etc... or persuade him that as you are not married he should leave the money direct to the kids, with them as trustees, for example.
As someone else pointed out, even if he did leave the house to you in his will, you would have to pay any inheritance tax due - i.e you could be liable to pay a hefty cash sum to the government for the privelige of staying in the house. As there is no inheritance tax between husband and wife, if you were married the gvt would not benefit from the investment in the house at that point.
Tell him that!
With a will in your favour you have some security for as long as that will lasts (and as I say, it can be changed within an hour), and IF he dies. If he changes his will, and does not go under a bus your actual security is zilch, zero, nada.
BooBoo - you know everyone is saying this because we are on your side?