Dating after divorce and teenagers
I can certainly understand why this makes you uncomfortable.
We even used to go to the movies just me, dad, and my brother but now its me, her, dad, brother and the girlfriend's son. When families are transitioning and making changes it is often tricky and difficult for everyone. We often wish for the old and familiar because the new situations make us feel replaced and uncomfortable.It sounds like you have enjoyed your relationship with your father for a long time.Your time alone with him and your brother was, of course, very special to you.It is also very important to tell him how his physical behavior with his girlfriend is making you feel.
It doesn't sound to me that you dislike your father's girlfriend.
Let's see if things can settle down so that you feel better. If his mom is still around and in the picture somewhere, it would seem natural to feel that his dad's attention should still be towards his mother if all was right with the world.