I talk to her sister on Facebook and she flipped out on me. What to do (kinda long and serious answers only)?

Sometimes people can get possessive over their friends. M is feeling insecure because she's been compared to her sister all of her life. M is probably also feeling like you invaded her privacy by developing a relationship with her sister behind her back. M feels like a third wheel in your friendship with V. I think the best thing to do would be to make an effort to become better friends with M rather than insulting her, even when she accuses you of mean things. Encourage her. Tell her that even though she doesn't write on your wall much, you look forward to seeing what she has to say. Remind her that you've been friends with her for a lot longer. Don't lose your friendship with either of them.