Some problem about LOA that need advise

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Not sure anyone could lend me a hand on this "letter of administration" issue I got from my dad recently, its about my aunt is still holding my late mum monies and she is my LOA when I was young time but unfortunately thing goes bad when I wanna my share back as my cousin (which is her son) say its been that long which cant be return and dont know where the money my aunt put..

So I wonder does anyone been before this problem could advise me what should I do?