I actually do understand this. But, I also agree professional help is needed.

Your sister is coping with the incest in her way. She is being promiscuous and acting like she is a 'grown up'.

You are stunted in your sexual development. Also, the way you handled your frustration and jealousy was not healthy. A lot of what you're feeling may also be a reaction to the incestuous sexual abuse your sister experienced.

Does your mother know your sister was abused by your brother? Is that even true?