Dear Hippy,
That's not only child abuse but also child endangerment. You and your sister have gone thru something traumatic. When parent doesn't behave like one, it definitely affects you emotionally.
Your dad is not behaving very responsibly by refusing therapy for you.
How is your sister holding up against that incident? You both should talk with each other about it...it would definitely lighten the load for both of you.
Talking to a school counselor is a great idea and both you and your sister should try it. School counselors are not for school related purpose only....they are there to help students in
any way they can.
If you can, please push your mom into therapy. She really sounds like a person who could benefit from therapy....maybe you guys can even use a lil emotional blackmail that you won't be visiting her unless she enters therapy. You may never know, she may actually start behaving like a mom to you guys.
Good luck! We're here to help!!