Shaymus,
While I understand that your girlfriend pays the rent the bottom line is that you are suppose to be the other adult figure in the house. If you are expected to take care of her children then you have the rights to make rules as well. The oldest child's behavior is unacceptable. I know that in CT if there is violence in the home that threatens a minor any T is a court mandated reporter...meaning that he/she has to report the violence to the Department of Children and Families. I do not know if the rules are the same where you live or not.
Have you talked to you girlfriend about how you feel? If not then maybe it would be a good idea.

Stay strong,
Jessica

"Take these chances. Place them in a box until a quieter time..." ~DMB