About your sister :
Your sister has acted like a bitch.

About you :
You don't have to waste you time with that.
You have done nothing bad, really.
Your sister is annoying, so you don't want to fend her. Sound logical.
You didn't hurt her, she hurt you.
You cut your arms for her (that's very fucking stupid. "I LOVE YOU LOOK ILL CUT MY LEFT HEAR !!!").

About your girlfriend :
She denounce you, dump her.

Again, give up, don't waste your time with that, it's not worth the prize.