Yes - and you would be in prison.
John B. Whalen, Jr., Esq. provides free initial home consultations seven (7) days per week, including all evenings, weekends, and holidays, from 7:00 AM to 10:00 PM. Mr. Whalen is an AV Peer Review Attorney and Counselor at Law, is listed in The Bar Register of Preeminent Lawyers, and is Avvo Rated 10.0 Superb. Mr. Whalen has spoken for the Pennsylvania Bar Institute and the Delaware County Estate Planning Council, and has had his legal articles published by the Pennsylvania Bar Institute, the Pennsylvania Law Weekly, and the Martindale.Com website, and he has had his law blogs published on the Lawyers.Com website.
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If you apply to be the administrator of the estate, you would be required to notify everyone who would be intestate beneficiaries under the law. If your deceased cousin had no surviving spouse, parent, or child, then his siblings (being the next-closest relatives) would inherit his entire estate in equal shares.
You say you grew up "as brothers" -- did your parents adopt your cousin? If so you may have a claim to his estate. But unless there's a legal adoption making you his brother, you would otherwise get nothing, since he has surviving siblings, who are closer in the intestate distribution scheme under the law than you.
What you're contemplating (not telling any of your other relatives) would be criminal fraud, and yes, you would be subject to charges, fines, and possible jail time (as well as restitution of all of the estate property you would take).
If your cousin wanted a different estate distribution than the intestate distribution law provides, he should have made a will. So many people think wills are unnecessary -- until a relative dies without one, and the survivors discover that the law of estate distribution is very different from what they might have thought.
I agree with both of the other attorneys. There is no way for you to legally wind up with this estate absent a Will leaving you the assets or joint property or beneficiary designations in your name. Your cousin did you a disservice, if he truly intended you to have his estate.
If you attempted to do this, you would probably wind up in one of the nicer prisons. But it is still not worth it.
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.