Kill her with kindness. Ignore the dirty looks, ask her how she is, and tell her that her hair looks great, or her outfit, or whatever, and ask her where she got them. Keep smiling. She has to answer somehow, especially if others are looking on. If she's nasty to you, it will reflect badly on HER, not you - after all, you've made the effort. Do not let her bait you, or upset you, but persist in being pleasant to her. Chances are, if you maintain a pleasant and friendly attitude to her despite her attitude to you, she will eventually either thaw out on her own, or else someone in the family will have a quiet word with her to get her to cop herself on.