Girlfriend's sister is a stalker?

A few weeks back in may my gfs sisters laptop had some problems so my gf that maybe i should look at it since i mess around with pcs. when i went on it and was clearing some of the stuff, i found some files with my name ***park.png ***first.jpg and stuff. being curious i took a look at it and found a lot of pics that were of my gf and me except they had her sisters face on it. seemed to be really badly photoshopped.pretty shocked then, just thought maybe im hallucinating or somthing so i delted the photos and just went on with my life. few days later im waiting for my gf but i meet her sister who said that something came up with my gf and wasnt able to make it. i later asked her about this and she said she didnt even receive my message to come out. another rainy night the sister stops by to drop off some food but doesnt leave for 3 hrs even after i hint that she should leave. I talked to my gf but she says that Im being delusional and just impressing my desires or something like that. shes not taking it seriously but im getting a bit paranoid, just changed my locks to be sure. should i file a restraining order or something? what should i do? i really like my gf atm so dont wanna anything to endanger this.