Your parents outgrew each other, but they still love you... they also deserve to be happy.
Your mom is on the rebound. Allow her to process through all this... things will calm down. Give her some space to explore her new-found freedom. Your dad is probably in the same mode.
If your mom's boyfriend tries anything with you or your brother, you will have to take some kind of action. I would suggest locking your bedroom door to begin with.
Actually, it may be a good idea to talk to your dad about what you are feeling.
What you can do is focus on your school work and other activities... while your mom's boyfriend keeps her out of your hair. The great thing is that you are old enough to make yourself scarce as long as you hold up your end of things... whatever chores you have to do and maintain your grades.
Take up some sports activities. Come up with some simple ways to make some money there in the neighborhood... babysitting, dog-walking, scooping dog poo out of yards (don't laugh at that one... you get several houses that pay you a flat fee to come in twice a week and pick up the doody and bag it up).
You can study at your friends' houses. Actually, your mom would probably welcome that as long as she knew where you were and you stayed out of trouble.