I will try and keep my answer as brief!!
For a start, you need to differentiate between mutual wills and mirror image wills. A mirror image will is just that. There is no legal force in the fact that wills of partners/siblings etc mirror each other. Mutual wills ARE uncommon as are contracts to make wills and I would be very surprised if the documents you have would go so far.
If they are only mirror image wills, then the estate is your step father's to dispose of as he pleases.
I take it he has now died then?
You could apply to the court for a variation of the will under the Variation of Trusts Act 1958. Your brother will need to be separately represented. However, you will need to show that it is in your brother's interests for your mother's wishes to be restored. Unfortunately, the court may well take the view that it is NOT in his interests to give away half of his inheritance.
If you are still within the time limit for an Inheritance (Provision for Family and Dependants) you could consider a claim for reasonable financial provision from your step father's estate. The fact that you were promised your mother's half of the estate will be something the court will take into account. Take some advice.