Okay OP, you want an honest answer?
There are two facets to your problem.
1) Your sister is underage
Read this thread:
https://incels.is/threads/atomic-jb...-females-potentially-sexually-arousing.72391/
It is scientifically proven that normal healthy men find prepubescent female children about 60% as sexually arousing as adult women. Thus we can conclude it would be within the normal healthy male sexual range to be capable of arousal in this way.
However, laws have been written banning sexual contact from adults with children. Whether anyone believes these are appropriate or not, they exist, so you can't do anything about it. Even pictures of underage girls can land you in jail for decades. So it's best to try not to think about it. Or jerk off to whatever mental fantasy and then let it go.
2) Your sister is related to you
Read this WikiCuck section:
https://en.wikipedia.org/wiki/Incest#Prevalence_and_statistics
Incest is incredibly common and was historically far more common. Thus we can conclude it is pretty routine for people to be sexually attracted to their relatives and this is not very directly abnormal either. Incest themed porn is one of the most common fetish categories.
Again however, because of point #1, you can't do anything though unless she is >18, and even if she is >18, you ought not to do anything because society still frowns on incest.
Outcomes
If you were to do anything that could be identified as incestuous or sexual towards her, she may not be directly harmed by this. I don't believe all sex with kids would automatically create trauma. When I was 10 I had sexual fantasies and would have had no problem if an older girl had given me the chance. I would have been eager for it.
However, she could easily blackmail you later, or ruin your life regardless of what she feels at the time. Even 20 years from now she could come out and say you "abused" her for something very minor and ruin your life and make your family hate you.
So in short, you don't do anything about it. You suffer in silence. You try not to think about it. Or if you do have to think about it, you fantasize mentally and jerk off and get it out of your system. You try not to let anyone know. And like I said, if this is starting already, you've got a long road of torture ahead of you, so you better get mentally prepared.