As I was reading your post a couple of things came to mind.
First, I was hoping that as difficult as it was to do, your laying it all out the way you did, brought you some peace of mind and it sounds like it did.
Secondly, as I read, I noticed that even though you are fully aware of sisters mental illness, on some level may be trying to make sense of her actions toward you and are still feeling the sting of the hurtful words and behavior. This is a pretty common and natural thing to do, especially when it's a family member. When there is a violent crime committed, the first thing people want to know of the accused is "what was he thinking...how could he do such a thing?". They are trying to use logic and rational thinking to make sense of the actions of a crazy person. You lived with your sister since childhood so her crazy behavior, as odd as it was, was part of your "normal" day to day life and could be part of the reason why it's so hard to get over. If you worked in a mental care facility and your sister was one of the patients there, you'd be able to dismiss everything she said and did and put it all down to her mental illness. Because as a child, you lived with it day in and day out, you couldn't always see it objectively the way an outsider could.
It's been a long goodbye and right now rather than mourning your sister directly, you are in a sense, mourning your childhood. You're feeling sorrow for your younger self and what you suffered at the hands of your mentally ill sister and suffering the loss of what could have been. On top of that, your sorrow is compounded by the loss of the consolation that family members can bring at these times.
I hope it helps to be reminded that all of this came about through no fault of your own. You didn't choose for your sister to be afflicted with mental illness and you didn't choose to be brought into a religion that causes otherwise rational loving parents to fear god so much that to save their own hides, they must turn their backs on their own offspring.
Memorial and Funerals are to help the living grieve and under the circumstances no one would blame you for not attending however doing so may give your family an excuse to resume their shunning. If you are able to attend and view it as a way to honor the memory of your sister and bring closure to and unfortunate situation, it may bring you a measure of peace in knowing that no matter what others do or say, you did the honorable thing.
In the meantime, you'll need to trust that time will be a friend to you in all of this and that you'll come to grips with all of the troubling feelings your are experiencing at the moment.