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Provided you purchased as Joint tenants, the Flat passes automatically to you, through survivorship (like a joint bank account). Therefore, your Mother's "share" in the Flat does not form part of her Estate and if for example she had left this share to someone in her Will, the legacy would fail, as the whole of the Flat passes to you.
Therefore, your brother is certianly not entitled to anything from the Flat.
I hope this clarifies the position to you.
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