What is the ruling on taking a portion of the social security allowance without the knowledge of the siblings and their guardian? And is one acquitted of a previous debt if the rightful claimant later relinquishes his share without the debtor's knowledge?

There is no blame upon you for taking your known and specified share of the salary and the annual lump sum without the knowledge of your siblings and your mother. As for taking the entire amount of the annual lump sum the first time, this is considered an encroachment upon the money of your minor siblings, and you must repent from that. Your obligation has been fulfilled by leaving them the full annual lump sum in the second year, and there is no need to inform them of what happened.