No. If your stepson was under your supervision when he set your curtains on fire, his mother is not liable to you. In California, only the parent who has custody or control over the minor will be liable for his willful damages he causes to the property of others. In this case, as a stepparent, the court will most likely hold you liable for the willful actions of your stepchild. Your stepson’s act of actually playing with the matches was the willful act, not the setting of the curtains on fire.