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):
I'm very sorry as that's probably not what you wanted to hear but I must be truthful in the information I provide.
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One final point. The above information is based on property law principles and predicated on the basis of their current entitlement.
If an application to the family court was made for financial relief (i.e. to decide how the assets should be divided) a family court would not be bound by these principles. Whether the court made an order for a 50:50 split of the property would depend on other matters such as a detailed analysis of the needs, ages and finances of the respective parties.
I hope that's helpful and don't forget to rate.