How can i prevent having the flu?

Eat well. Rest well. Avoid stress. Wash you hands often. Do not touch your face unless you have just done so.

drink water and wash ur hands

a) go get flu shots - a seasonal flu shot now and a swine flu shot when it's available. They won't help right away, but this is probably not the last time you'll be exposed to someone with flu this season. b) wash your hands or use hand sanitizer a lot, especially after touching things your sister has touched. Don't eat/drink out of her plates/glasses. Don't share towels with her. Avoid using her pencils/pens. If she's old enough, she should cover her mouth when she coughs; preferably cough into her arm/sleeve. Do be nice to your sick li'l sis, though, because someday it will be your turn to be sick and it'll be payback time then.