Hi, thanks for your question. My name's XXXXX XXXXX I'm going to assist you with it.
If half of the property has been transferred to your stepmother and she is on the title deeds (i.e. by registering the transfer with the Land Registry), I'm afraid it is very likely that she is entitled to half of the property. In fact, if they own the property as "joint tenants" (rather than tenants in common) and anything should happen to your father, she would be entitled to the entire property.
Therefore, if this is a joint tenant and they have permanently split, it might be adviseable to "sever" the joint tenancy whilst you are negotiating buying her out. This is done by the party (wishing to sever the tenancy) serving a notice on the other party and then registering this at the Land Registry. In this sceanrio your father would serve it on your stepmother.
Details of severing a tenancy can be found here (particularly para 4.1):
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