You'retoo young to so angry and sounds like you know that. Why are you so mad? You shouldn't let your sister or her friend get you to the point that you get out knives.....instead go outside and take a walk...find ways to deal with your anger. Your age does have a lot to do with how you feel...I have a 13 year old and an 18 year old.
Please don't try and hurt yourself, please respon if you want to talk. Sometimes that helps.
Take care and God bless!