Does my girlfriends sister have a crush on me?

so basically we talk alot on msn, as my girlfriend is in Europe for a photo shoot and thats how we talk when she goes. yet her sister has been joking about spying on me and her sister, we talka bout sexual things in funny ways yet sometimes she carries it even after i tell her i feel weird. my partner even suggested the idea she might have a crush on me. now the plot thickens so my partner looks up to her sister so i am worried that she might worry about me sleeping with her sister, which i honestly wouldnt not even if my partner was keen, so what do i do? i cant confront her on mere speculation she has a bf who she has been with for four years. so what you think?