You should consult with a probate attorney right away. Your brother is an "interested witness." Unless there are two other witnesses to the will who are disinterested, the fact that the will makes a devise to your brother creates a presumption that it was procured by duress, menace, fraud, or undue influence. (California Prob.Code § 6112.) Unless your brother can disprove this presumption, you should be able to have the provision granting him the entire estate undone.
Interesting situation. I assume that your brother has now changed his mind about leaving the entire estate to you? As Attorney Weiss indicated, your brother has a real problem with the Will because he was an interested witness. If he persists in trying to claim the entire estate, I believe that you WILL need to hire a probate litigation attorney to assist you with this. Perhaps you can both agree to simply split the estate, however, and save both of you thousands of dollars in legal fees.
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You should consult with a probate attorney right away
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