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In circumstances like this, it would be normal for there to be a clause in your Mother's Will granting your broter the right to remain living in the property for as long as he wishes/for a certain amount of time, and then as and when he vacates. the property be sold and the proceeds split 4 ways.
Unless the above clause is contained in your Mother's Will, you are correct in that legally, your brother will not legally be entitled to remain living in the property unless the 3 of you agree to this- if the Will merely states the Estate is to be divided 4 ways, then each beneficiary is entitled to receive their entitlement and the Executors are under a duty to deal with the Estate efficiently, to include selling the property.
I hope this assists and sets out the legal position to you.