“I had to come over here and ask something quickly. Girls seem to know more about these things than we do… Okay I’m going get straight to the point… I’m REALLY REALLY attracted to my sister… How do you think I should go about telling her and my parents about this?”
Wait for the girl to give you a look of utter shock and disbelief then change topic and try having a normal conversation afterwards.If she is calm about the whole situation then smile and say
“That was all bullshit but since you handled that story… You might just be cool enough to handle me” *sly smile*