Is the questioner burdened by sin or iniquity for having caused his mother's death, after informing his maternal uncle of her egregious act, which led to the uncle killing her?

If you informed your maternal uncle to stop her without alluding to killing her, then there is nothing upon you. Increase your supplications for your parents, and be keen on raising your younger siblings and protecting them from deviations. There is no sin upon you for what your parents used to do, as long as you advised them, for "No bearer of burdens will bear the burden of another," and "Every soul, for what it earned, will be retained." What they do does not negate their right to dutifulness and kindness. Dutifulness to the deceased is achieved by maintaining ties with their kin and honoring their friends. Strive to advise your father and turn him away from bad companions and treat his addiction.