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The proper safe would be anything that has a combination lock and where the guns are not visable. For instance the safe has to be made of steel or iron, etc. It cannot be a glass cabinet. I suggest your Father replace the gun cabinet with such a safe and go to your brother's Probation violation hearing and show pictures of what is now securing the firearms.
He can explain that your brother has never come into his room and never had contact with the guns. And that he had no idea the cabinet was not satisfactory
Your brother can attest that he has been compliant in all of his probation conditions.
That is all you and your family can do. You can also contact the PO and tell him a safe has been purchased and request that he vouch for your brother's complaiance in all other matters concerning the conditiions of his probation
Unfortunately, it is a violation when there are guns that may be obtainable by the offender.
So you need to show they are now secured with a safe and combination that your brother will never have access to
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