No matter how many times we say, people go on doing stupid things.
If you are going to have sex, either you need to wear protection to prevent against pregnancy, or your partner needs to be using some form of of contraception, either the daily mini pill, a contraceptive implant, or an intrauterine device. This is mandate three, not an option!!!
Abortion should not be considered as just another method of contraception. It isn't. It is the murder of an unborn child, and there are very few reasons why this should even be countenanced. I'm not often disgusted by people's attitudes, but the idea that you can casually have sex with someone, then tell her that she has to have an abortion if she has by some stupid means, got pregnant, shows such a callous disregard for human life that it actually sickens me.
While the chances of you making your sister pregnant are fairly small, you have to understand something about human anatomy, and the reproductive system. If you didn't know this before you started having sex, then you shouldn't be having sex in the first place. The lubricating fluid that is emitted from your penis before ejaculation, commonly known as pre-come, is likely to contain some sperms. It only takes one sperm to make a girl pregnant, but given that the average ejaculate contains anything from 2 million to 20 million individual sperms, and even that number is not always enough to get a girl pregnant, then you have probably avoided hitting the jackpot. But notice I say probably: many an unwanted child has been born because of stupidity, and that stupidity includes people who think that pulling out before ejaculatory is sufficient to prevent pregnancy.
You say you recently had sex with your sister. What do you mean by recently, like yesterday evening, three days ago, two weeks ago? If it was anything more than last night, then the emergency morning after pill is unlikely to be effective if she did get pregnant. You really are a stupid individual, and like most guys, you thought with your cock instead of your brain.
You had better make plenty of offering to your God or gods in the hope that you and your sister have avoided a disastrous and costly mistake, and if you haven't avoided it, and she is pregnant, there's no such thing as a natural abortion, it's something that is done by medically trained professionals. In the early stages, the fetus can be aborted by a cocktail of drugs which are only available through a proper Dr. If she is pregnant, you had better take responsibility for your actions, and at least go with her to the doctor to support her a time when she will feel really emotional and upset. Taking the drugs may have side effects that make her feel weak and nauseous, and she will need somebody to support her. You think you're man enough to have sex, then you'd better be man enough to accept the consequences of it.
If you are lucky enough to find that she is not pregnant, and the two of you wish to continue having sex together, then forgot sake grow up and be sensible. Either go with her so that she can seek proper contraceptive advice, and start using whatever appropriate method she decides upon, or you go out and buy a condom is, and stop having stupid ideas about putting in without protecting her. As I say, if you want to be a man, you have to accept a man's responsibilities!!!