My girlfriend hates my little sister?

My girlfriend 17 she is terrified of my little sister and she is only 8 i introduced her to my girlfriend and she wa s the sweetest girl ever. I let my girlfriend and little sister spend quality time together and i let my girlfriend babysit her and all that i sometimes blew off things i was supose to do with my lil sis to hang out with my girlfriend and my girlfriend is telling me that my little sister is trying to kill her. I mean an 8 year old comon. Ive known my litle sis for a long time she is nice. Right now me and my girlfriend was supose to study but i went upstares to change my shirt and my girlfriend was on the ground and so i asked her what was wrong and she said my little sister grabed a pot and started wacking her with it pulling her hair trying to choke he with a jump rope. So i called my sister down from her room and asked her what happend and she started crying and i told her it was ok amd to give my girlfriend a hug and my girlfriend goes crazy running behind me yelling ''keep that little demon away from me