I agree to try the e-mail to your mom first, and if that doesn't work then you need to get other adults involved. Maybe tell your therapist if you have one. If you don't then you need to talk to a trusted teacher or someone at your school, or another relative if you have one. Maybe not your dad, but someone your mom respects. Tell them that your brother is hitting you and your mom doesn't understand, she thinks it is normal sibling fighting, but it's not. Maybe hearing it from another adult will help. Your brother is exhibiting abusive behavior.