I don't necessary think that scaring people with yukky pictures etc will do the trick, because time after time, research shows this type of education doesn't work. It doesn't work, because young people cannot believe that this type of thing [insert yukky picture] will happen to them. They believe they are infallible, and it will be others who will be unfortunate, not them. What does work is providing simple information and sign-posting toward services, so that if there is a problem, they know where to go to get confidential advice and help.
I think the best thing you can do with your sister is to give her a basic overview about contraception - particularly emergency contraception, and how important that if she needs emergency contraception, the quicker she gets it, the better it will work, and this is where she can go to get it in your local area [give local family planning leaflets]. Also describe some of the basic STI's, most importantly Chlamydia (as this is statistically the one she is most likely to get) and spend a lot of time explaining how you can have infections but not know, even if you have no symptoms. Then give info about local STI clinics where she can go to get checked out if she ever has unprotected sex. I think as soon as you start mentioning things like HIV and syphilis, most young people turn off to the info you give them. I agree that it is important they know about these things, but unfortunately too much emphasis is placed on infections that (statistically) they are unlikely to get, and therefore, because they don't know anyone who has these infections, they believe they won't get them too.
I think if during your talk she is scared with pictures and over medicalised lingo, when she does make a mistake in the future(and don't we all) she will be frightened to come to you when she REALLY needs help, especially in an emergency. Keep it simple, give her local services info, and let her know you are always there for her if she needs you in the future.
Its great that you have such an open relationship with your sister!