Somegirl,
a few years ago I attended a 5 days retreat/workshop for people who had been emotionally or physically abused as children. The people who attended covered a wide spectrum of severity of their abuse. Although I was not sexually abused, certain events in my childhood had a very detrimental affect on me growing up. I found the workshop very useful and I released a lot of pent up emotion, a few months later I addressed my alcohol problem and have been sober for 2.5 years now.
I have no answer, I can only relate my experience but there is help out there, what you are carrying around with you is very real, your resentment towards your sister, it's making you unwell. You need to be heard, listened to with compassion.
I hope you can find that help
caiHong