Most Helpful Guys
Those are pretty weird looking. That could be enough for me to not ask her out. She is pretty otherwise but those things look kind of freaky.
It's your sister so go ahead and tell her. It's better to hear it from you than someone else. Don't go up and say "you're eyebrows are bloody hideous" because that is obviously too blunt but instead say "Sis, I think you should maybe lighten your brows a little bit... here, let me show you how you can make them REALLY nice and cute". Something along those lines. Instead of telling her how bad they are, tell her how she can make them better.
Most Helpful Girls
Tell her. She may be mad at first but she will thank you later