Sister's friend hitting on me and I have weird thoughts

So, My sister has a friend (2 years elder to me). I've known her for quite some time now. Recently, she has started texting me without any purpose. I get a feeling that she's trying to hit on me. Idk what to do.

The thing is she's very pretty, I don't feel anything for her tbh, but I am getting some indecent thoughts.

what a terrible place to be in, man. can't even do anything about it cuz I don't want my sister to know if I fuck this up.

ever been in a similar situation?