Everyone makes mistakes, Relix. The problem is that you don't seem to learn from yours. Obviously the guilt isn't bad enough to get you to mend your ways. If this girl is half as decent as you have described, you do not deserve her. Period.
The problem here is that she doesn't deserve to be hurt by your actions. Also, nobody be fooled, the sister is as guilty as you are in this situation....... and as painful as it will be for your EX, she does deserve to know that she's surrounded by snakes, disguised as loved ones. Hopefully it doesn't destroy her and she can push forward.
Being cheated on is one of the most terrible things someone who is in love can endure, this I know. Time spent in secrecy only makes it worse- personally, I couldn't believe I was so stupid and naieve to not see the (now obvious) signs. It ruins one's trust in humanity..... at least for a while.
She deserves to know the truth. You are young, she is young, and her sister is young. This will leave a scar, on all 3 of you. Another lesson of life learned. Do not forget this one, Relix, and hopefully time can heal everyone involved.