I’m sorry that you feel like the “odd man out” at home. Please know, however, that this is not unusual in blended families. You may be insecure about being biologically unrelated to your stepfather. Therefore, you perceive the things that he says and does as slights and get easily hurt by them. On the other hand, your stepfather may be unconsciously showing favoritism toward his biological daughter, oblivious to the pain that it’s causing you.
I encourage you to have a heart-to-heart talk with your mother, explaining how you feel like an outsider in the family. It’s important to open up and be vulnerable. She may instinctively defend herself and her husband at first, claiming that the problem is all in your head. Hopefully, though, she’ll eventually understand how much this is bothering you and want to make it better.
Working with a family therapist (online or in person) would be beneficial. A good therapist can help the four of you see the dynamics that put you on the sidelines. He or she would give suggestions on how to change this. A professional sees things more clearly than those who are emotionally entangled and can be incredibly insightful because of their objectivity and expertise.
Our families should bring us comfort, joy, and security. Since yours is not doing that, please be proactive and say something to your mom. They can’t make it better until you tell them how you feel.