First of all get a copy of your Dad's will and see what it said
If your parents have made mirror wills then your step mum's will is not available as she hasnt died
and it all depends if she was left the house absolutely, in which case she can do what she likes with it
There are as you can imagine circumstances where someone is left something on condition that they do something - in this case divide the house in a certain proportion and you will not be surprised to learn that if the wording is right then the first will can be enforced
http://www.birketts.co.uk/resources/legal-updates/1371/the-differences-between-mutual-and-mirror-wills/
so it boils down to .... what your dad's will says