Just to let you know in my case i was told that a big reason behind my wanting to act out with my sister was because of family problems sometimes we need to feel close to family and if our family is messed up we try to implement 
so to reiterate what i said earlier you are NOT a slimy monster
(tip to mods maybe you should make a CAPTCHA I am not a slimy monster)

True maybe your sisters challenges are from family issues, maybe they are from the story you mentioned, maybe a combination of both, maybe something completely different. The best way to find out what is the cause, is to speak to an expert/psychologist.  

Best of luck Imganadoit, hope I am not bugging you too much.