Any Will can be contested. That said, it is not easy and is usually an uphill battle. The grounds for contesting Wills are generally: 1) failure to properly execute, (Will is not signed, does not have the right number of witnesses, etc); 2) the testator lacked capacity, (did not know what he was signing); or 3) there was undue influence exerted, (testator could not freely sign Will, because of threats of violence, etc).
The burden of proof is on the person contesting the Will. If the Will is contested, you would need to hire an attorney to help you defend it.
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Agree with Mr. Frederick, contesting a will is an uphill battle. Your question suggests that if your brother's intent was to "cut out" all his relatives except for you, and he named them all, someone was doing things the right way in drafting his will.
That's because, it's usually presumed as a "default" that people love all their relatives, so if the will is silent, the property will be equally divided among his closest next of kin. But if the decedent drafting a will says specifically, I intend to leave "W", " X" and "Y" nothing (or a nominal bequest like $100 or $1000 which is the polite way of doing it if you really don't totally hate those you are "cutting out" or want to make them so angry they will contest the will) and the remainder of my estate to "Z" (you in this case), the only issue is whether the decedent was of "sound mind" (not delirious or suffering dementia) when he signed the will and said to the witnesses that this was what he wanted to do (that is, he wasn't "out of it" and you were putting a pen in his hand and he had no idea what he was signing, which some unscrupulous relatives or caretakers sometimes attempt to do).
But do get a good probate lawyer to fend off possibly angry siblings who may think about contesting the will in probate proceedings.
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