What should I do after losing most of my brother's money – which he inherited from my father – after investing it in stocks with the intention of developing it, knowing that it is impossible to compensate for that loss?

If the younger, adult brother gives permission to invest his money, or if you are a guardian over your younger brother's money and wish to grow it, and the invested money is lost without any transgression or negligence on your part, then you are not liable to guarantee it. This is because you are entrusted, and a trustee is not liable unless there is transgression or negligence.