From yet another perspective...
I went to my mother when I was younger and told her I was thinking about having sex. I asked her to help me get some form of birth control. She told me absolutely not as she was not going to "give me permission" to have sex by helping me get birth control. I had a baby six months after I turned 16.
I wish I would have had someone to help me out. I was going to have sex weather mom said it was ok or not. When she said not, even though I knew the risks, I still had sex but also had a if she doesnt care why should I attitude about birth control.
It's not about age. I understand helping your sister could put you in a precarious position with your family but consider the alternative. If she was going to talk to your mother about it she would have gone to her instead of you. Adults these days tout morals and values and praise themselves for progressing and offering resources to our young but pull their heads into their shells when our young actually utilize the resources we always tell them are there.
Talk to your sister. She is the only one that is going to be able to make up her mind and nothing you say will make it so she does or doesn't have sex. I think she has shown some great maturity in asking for help in the first place. She also needs to be informed that birth control is not std prevention and given information on that as well. Maybe, if the choice to help out or not all comes down to how the parents will react...let your sister know how you feel about it and offer to talk to your parents WITH her. If she is adamant that she will not talk with your mother leave it at that. You could lose the trust she obviouly has in you in the first place. IMO I think you should help her or at the very least, direct her toward planned parenthood.