Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
Do you still have a copy of your father's Will?
Did his will go through the probate process?
Are you aware that the site charges more for a phone call
Yes it would - and that means you can get a copy of the Will using the site here
What your are looking for is any wording that suggests that your father and step mother were making Wills at the same time that were meant to be "mutual" wills - basically unchangeable.
If you know which solicitors acted for him in preparing the Will they MAY still have a copy of the notes - and you can request them quoting the case of "Larke -v- negus".
You can challenge the Will - but to do so successfully you need to have some evidence that there was a clear understanding that the house was given to her on trust that she shared it between all of you.
In the mean time you can file a Caveat on your step mother's Will to prevent a Grant of Probate being made - which could enable you to try and negotiate the matter with your stepsister
I hope that this is of assistance - please ask if you need further details