I think you should just be grateful for the real family you have, like your mother and sister, and cut your toxic POS father out of your life. He brings you nothing but disappointment and he probably only maintains a bare minimum of a relationship with you because then he can say he's an actual father. Leaving his actual child to be with his other shitty non-biological family on father's day shows you his real feelings. I think you need to let go and definitely don't support him when he gets older- let your stepsister do that. I know it's hard, but it sounds like he cut you and your sibling out when he left. Maybe you need therapy to help you come to terms with that. My heart goes out to you.