I hate that you're going through this Amy. You have fought your demons with addiction and now are being forced to sit back and watch someone else to fight the same battles. I ask you what advice would you give if you read this post from someone else. Is there anything you think they could or should do? We all know these answers but living them out is so much harder then typing them out on a computer. Until your dad and stepmom are ready to deal with the real issues then anything you say is not going to get through the wall of denial that they have put up. You have had your battles and they will have theirs. When they are ready to address them you can share what you have learned through recovery but until they are ready it wont come across in the loving manner that you are intending. As everyone has told me, the addict has to hit their bottom - your stepmom and dad have obviously not really hit their bottom. Your family knows what addiction is and i would suspect that part of the reason they are snapping so much is becuase they know that they have a problem - they are just not ready to face it head on yet.
Even though this affects you a great deal, this is their lives not yours. Your father loves her and wants to believe her even though the redflags are all over the place - he's choosing to ignore them right now but with how i've heard you speak of him I dont think that he will stay in this denial stage too long - when he wakes up he will need you and you will be able to help him understand this disease better. Until that time distract yourself and do your best to step out of their circle.
How is your neice handling all of this? I worry about her being so young.