Okay..I'm sorry to hear that you've recieved so many bad comments.
When you slept with your sister during high schoool, did you love her and did she love you?
My advice is, that if you want to continue this with your sister, that you keep it quiet if you don't want to hear people saying things to you.
If you feel bad about what you've done with your sister, than calmly talk to her. I'm not experienced with this, but still I am sorry that people have reacted this way. My only other advice is that you think things through before you act.