You need proper and detailed legal advice here;
Various scenarios may apply; the "perfect" scenario is if the house is still in your mother's name, if she left no other real property, and if you and your sibling are her only children. Then, your step father could simply disclaim his inheritance and it would pass by default to the two of you. No stamp duty, no C.A.T. (unless you guys inherited other stuff from parents);
It is very important to get proper advice; if you haven't already spoken to your own solicitor, do so without further delay