Regardless of your feelings for your sister, and her feelings for you, any time that two related individuals engage in sexual activity it is called “incest” and it is considered a felony. While the laws regarding incest and marriage between relatives vary from state to state, in general, situations involving first-degree relatives, like brothers and sisters, or parents and children, are considered illegal in all cases.
In theory, an incest conviction can result in serious penalties, including fines or imprisonment, regardless of whether or not the relationship was consensual. However, in most cases, incidences of incest are not reported, and thus they are never prosecuted. So, in your case, chances are no one is going to press charges against you and your sister for being sexually involved, but it is a risk that you should be aware of and willing to deal with if it should arise.
In terms of health risks, there is no additional risk to having a relationship with your sister, aside from the problems associated with pregnancy that you have already considered. Keep in mind that birth control pills are not 100% effective and their efficacy diminishes considerably if they are not used correctly. Please remember that there is a significant chance for birth defects in children born from the union of first-degree relatives.
We do live in a society where having sexual relations with family members is seen as inappropriate and the repercussions of having your sexual involvement with your sister exposed might be very troubling and difficult to deal with. They could even end up driving you and your sister apart. Even though you and your sister have been involved for two years, you are still only 18 and 16 years old. You did help her get through a rough time in her life, and sometimes sharing hard experiences with someone else can make you feel even closer to them than you would normally feel. And, sometimes, this closeness gets expressed as sexual intimacy. FX does not want to discount the feelings that you seem to have for your sister, but wants you to remember that you are still young and going through many changes in your life. Things like considering a vasectomy at age 18 is a very big decision to make, as it is a permanent procedure and could cause problems later on in life should you be in a different relationship.
FX thinks that you should talk to a therapist, psychologist or psychiatrist about your relationship with your sister. It would also be very important for your sister to be in therapy as well. The therapist will be able to help you both deal with the complicated issues you are facing and to sort through any mixed emotions you are experiencing. While you may feel outsiders may not understand your relationship, it is important that you realize that it is unhealthy for the both of you and bound to cause emotional turmoil down the road.
TeenHealthFX understands that it will probably be hard to talk to a professional about you and your sister’s relationship, but it is important that you do so. If you live in northern New Jersey and need help finding a therapist you can call the Access Center from Atlantic Behavioral Health at 973-247-1400. Outside of this area you can log onto the US Department of Health and Human Services Substance Abuse and Mental Health Services Administration website for referrals in your area.