How do you keep your 15 year old brother from getting between your mom and dad's fight?
Sadly, there probably is not much you can do to stop him. You could attempt to talk to him and convince him not to interfere, but it is not likely he will listen to you.
if you have an older brother then he will have most of your mum and dads money when they die but if you feel left out try talking to your dad about it or your mum but if you are still left out call a helpline for some advice still left out you can sadly try to get rid of your brother but not recommended.
The best way to get rid of a bad girlfriend is to tell him how you feel about her. If he still feels greatly about her, have a good conversation with her. Get her to tell you about herself and why she likes your dad. If you still dont like her, make your dad think she is not telling him somrthing to get them in a fight. Good Luck!!