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I am guessing you have a "Restriction" on the title, as opposed to a "restrictive covenant". A Restriction will indeed prevent your Brother from selling the property without your consent. Indeed, his Solicitor will need to contact you and the buyers Solicitor will insist that they see evidence that the Restriction has been complied with, before exchanging Contracts.
It will depend on the wording of the Restriction, but normally, you would sign the appropriate form to confirm the Restriction has been compllied with, IN RETURN for you receiving your one third share.
As there is a lot of money at stake, I would suggest that you contact a local Conveyancing Solicitor sooner rather than later to guide you further.
I hope this assists and answers your question.