Is the questioner considered negligent and will he be held accountable before God for abandoning his father's trade, which led to debts after he left it to them, despite suffering from pangs of conscience for achieving a better situation than them after great effort, knowing that he helps his younger siblings with a monthly sum and resolves inheritance issues?

There is no blame upon you for not managing the trade and wealth belonging to the heirs, unless your father had entrusted you with the care of your younger siblings or the judge appointed you as their guardian. If your siblings are in need of financial support and you are affluent, the preponderant opinion is that you must spend on them in a reasonable manner. Aiding your siblings with money and resolving their problems is a righteous deed, among the best of deeds.