Your Mom is your Dad's 'next-of-kin' if they were married at the time of his death. Most states have specific rules as to 'who-gets-what' and normally a surviving spouse would be the heir unless that second spouse dies. I'd recommend you meet with a Lake Charles Estate Planning attorney and get this resolved. Good Luck!
Louisiana laws on Succession are entirely different than any other state. In order for your mother to get the vehicles in her name, she will need to open the succession in probate court. I suggest finding a succession attorney as soon as possible. Depending on the size of your father's estate, the process can be lengthy at times.
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Ms Garland is correct in stating that you need to open the succession. If there is not much property, the succession can be completed rather quickly. Still, if one of your siblings is not cooperating, then the process will likely drag on.
You should find a local attorney in Lake Charles.
Best of luck.
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