"My problem MAY be, though, that once I do the letters (which of course I won't send to my sister), I will forgive her so much I would let her back into my life, and it would all start again. "
You might have forgiven her on paper but you haven't cleaned up the abuse pattern in your brain!!
I though I was able to handle my sister - convenient distance and wisdom but I now realized that I had internalized an abuse pattern and chosen a partner just like her. A man with huge rage at his mum and is kind, helpful on the outside but given an opportunity will unload his tortured goal on you or his son.
So the three letters might be the thing to do, went your rage, realized why she build up her rage and forgive her, and finally most important accept and release the rage/abuse pattern in your brain!
This is an interesting side to read
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