While it may be difficult to see your mother falling in love with another man beside your father, it is something you will have to come to grips with. With your mother remarrying and step-parents coming around, it's always hard to handle; however, they need to be happy as well.
My mother remarried when I was ten years old, and my father has been with the same woman for several years now, and while at first i felt a lot of jealousy, someone stealing my mom from me, someone besides my father trying to raise me, I did eventually learn to love the man my mother was with. While you may not feel like it's the right thing for you or for them, as long as that person is treating your parent well, there shouldn't be any negative feelings toward them.
I hated my stepfather at first. However I have grown to love him very much over the years. The important thing to see is that your mother remarrying and step-parents coming into the picture isn't meant to hurt you, but to help. Often times the child doesn't remain in contact with one parent, so sometimes when a parent remarries, it provides that parent role that the actual parent isn't filling. You must look at it from your mother's perspective: she wants to love somebody and wants someone to love her as well, and it is a love different then that of a child for their mother, so it is something you cannot fill for her. You need to let her have a man in her life who helps out, loves her and makes her happy, and also treats you like one of their own children and respects you.
It may help you to have a conversation with your mother. That way you get a first-hand view of what she's feeling, and remember that this new person in her life is not going to replace you, it will simply be a pleasant addition to the family. It would also help you to get close to the man that she is marrying, get to know him and see how he is like. Even though it may be hard to do I'm sure you will realize, as I did, that mom picked a good one and that you, too, can grow very fond of him. He may not become a replacement for your father, just another man playing a fatherly role in your life, which doesn't hurt.
In my case, however, this man was replacing my father because my father was a good man, so once I got over the anger of my dad not being around, I learned to accept his love, that he was being genuine and he did care for me as one of his own children. It can be especially difficult to handle your mother remarrying if the man she is marrying also has children. In this situation it is very important not to retaliate in anger, but try to be kind to one another, because whether you want it or not they are now also a part of your family.
Having more children around to play with can be a wonderful thing, but if not, as hard as it is to deal with having new sibling. Keep in mind that it's very difficult on the parents as well, because they are also taking on the responsibility of extra kids. Just be kind with everyone in order to make it an easy transition and you can learn to genuinely care for all the new people that have come into your life. If you simply cannot come to terms with this situation, at least try to seem accepting for your mother's sake because as a human being, she needs happiness as well.