Thanks for your question. I will try to help.
Where two people own a property together, they can decide to hold as tenants in common whenever they wish.
Where one person wishes to change and the other does not, the one who wants to change can do it without the consent of the other person. They can change to tenants in common even though the other person does not want to change.
This is the same for all properties in the uk, it’s completely normal. All that has to be done is the person wishing to change would have to complete form SEV from land registry, then serve a notice on the other owner and then apply to change it to tenants in common at land registry.
So, he can do this whether or not the other brother agrees.
If your mother’s Will express that they must hold the property as Joint Tenants then this would be unusual but it would not stop the brother actually being able to change it at land registry.
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