What are you being left? Nothing? Something?
Why is he doing this?
Is he of full mental capacity?
Is the half brother same father but different mother?
It would help us if you could explain the situation in detail please.
You can’t make your father change his will but you can challenge it if he doesn’t make adequate provision for you, a child, in his will. That may not actually be 50% or an equal proportion with your sibling but it needs to be an adequate portion. The statutory provision is the Inheritance (Provision for Family and Dependents) Act. It doesn’t matter that you were not on good terms or even estranged.. There is a very limited time window to bring claims under the Act of six months. You can only bring the claim after the death. You can also contest a will if you can prove fraud, undue influence from a third party, or lack of mental capacity. If your father had promised you something (rather than promising your mother) you may be able to enforce that promise if you relied on the promise to your detriment. However, he did not promise you, he promised your mother so that doesn’t really apply.
It is only the Inheritance Act which appears to apply to your circumstances.
There are lots of articles on the Act and here is just one https://www.hughjames.com/service/contested-wills-trusts-and-estates/inheritance-act-claims/
If you get with your half brother rather than have the argument and potential litigation, he may simply agree to ignore your father’s wishes and divide the estate with you. You know him. I don’t.
Can I clarify anything for you?
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Inheritance Act claims are not easy to bring but there are lots of firms on the Internet advertising for this work. However they do have a good success rate and the majority of claims settle out of court purely because they do have a high success rate particularly with straightforward circumstances like yours when your father has simply written you right out of the will for no logical reason.
It is certainly worth investigating.