Is it obligatory for the wife to observe hijab in the presence of her husband's stepfather (her daughter's father-in-law), who is not her husband's biological father? And is he considered a mahram to her?

Your daughter must observe proper covering and hijab in front of her mother-in-law's second husband, because he is a mahram to her. She is only forbidden to her husband's father due to Allah's saying: "And the wives of your sons who are from your own loins." Thus, a son's wife is forbidden to his father and grandfather.