My little sister is ually active?

I'm 25 and my sister is 17. My parents are very traditional, and will FLIP if they find out what she's doing. She had a pregnancy scare and I think she needs to go on the ring/pill. The problem is that my mom would definitely be more angry if she found out I was getting this for her, than if she actually got pregnant. The sense of betrayal, and that my sister went to me instead of her... Eesh... Advice anyone?