Just to elaborate on what others have said: this appears to be a case where the "Westermarck Effect" has not happened. This is a process where two people who spend their early years together (e.g. siblings) do not experience sexual attraction later in life in order to prevent inbreeding. If the siblings are raised with limited contact (as with your case), this effect may not occur, meaning that sexual attraction is possible. It could be that you find your sister so attractive because we tend to choose partners with similar characteristics to ourselves, also you clearly have a close relationship and you could be attracted to her personality. Conclusion: your situation is understandable but NOT NORMAL. DO NOT DO ANYTHING.