I strongly urge you not to run away; not only will it solve nothing, but you’ll also open yourself up to serious difficulties and dangers, as any police officer will tell you.
Instead, I urge you to do all you can to make this situation as happy as possible, not just for you, but for the whole family. You say you don’t think your stepfather likes you, but what have you done to build a relationship with him? Have you been sullen and bitter, or have you let him know that you’re pleased your mother is now happy? Or have you done everything you could to make friends with your new siblings? This may not be easy at first, but make an effort instead of waiting for them to take the initiative.
The most important thing I can urge you to do, however, is to discover a new family: the family of God. Elsewhere in your letter, you indicate that your family isn’t religious, but God knows all about your situation, and He loves you and wants to help.
Begin by asking Jesus to come into your life. Then ask Him to help you find a church where you can make friends with others your age. They’ll help you grow closer to Christ, and encourage you as you face problems. The Bible says, “And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another” (1 John 3:23).