I agree with everything that has been said regarding keep it simple. Just because someone is your sorority sister does not mean they have your back. Does it mean that you'll be "sisters" for life, etc? Yes. But that is not the same as having your back and being there for you.
At present, you have no reason to tell them the whole story, because as @Lifeline said, they have not "earned" it. Until someone "earns" it, just tell them you don't want to get drunk. End of story. And if they don't understand and pressure you once you've said that, then chances are high they won't have your back should you tell them your whole story.
I say this as someone who drank a ton in my 20s and tried to coerce others to do the same many times. I did because I wanted them to be as messed up as me, not because I cared about them as a friend. Those that I honestly cared about, I respected their wishes, whatever they were, without needing a full explanation. When you're drinking a lot and getting drunk, you end up (over time) having friendships that are based around alcohol and little else. It wasn't until I stopped getting wasted until I found out who my real friends were... everyone else fell away and got drunk with other people until they either wasted their whole lives or wised up.
At present, you have no reason to tell them the whole story, because as @Lifeline said, they have not "earned" it. Until someone "earns" it, just tell them you don't want to get drunk. End of story. And if they don't understand and pressure you once you've said that, then chances are high they won't have your back should you tell them your whole story.
I say this as someone who drank a ton in my 20s and tried to coerce others to do the same many times. I did because I wanted them to be as messed up as me, not because I cared about them as a friend. Those that I honestly cared about, I respected their wishes, whatever they were, without needing a full explanation. When you're drinking a lot and getting drunk, you end up (over time) having friendships that are based around alcohol and little else. It wasn't until I stopped getting wasted until I found out who my real friends were... everyone else fell away and got drunk with other people until they either wasted their whole lives or wised up.