Thank you all very much for your wise advice. Our friend's letter was a bolt from the blue and reduced me to tears - he put it inside my birthday card! - and I think I'm going to have to write back along the lines you have suggested. My daughter, the one targeted in the original argument, has said that she will support my decision either way but is worried that, if my sister came back into my life, all the unhappiness I dealt with during the counselling could come flooding back. Her older sister agreed and, having read our friend's letter said, "He seems to think she is graciously forgiving you - tell him to f**k off." I wouldn't go quite so far, but I do wonder whether my sister asked him to intervene or if he has just decided to act on his own initiative. I suspect that she has made him feel he has to do something, without actually asking him outright - she is incredibly good at manipulating people.
Eloethan, I have asked myself many times whether she was abused herself, and I have talked about it with my counsellor, and I have come to the conclusion that I don't think she was. I remember, when I was small and I asked her why she did these things, she told me that she got the idea from watching me touch myself when I was a baby. As an adult, of course, I understand that all small children explore their own bodies out of curiosity but, when I was 5, this just made me think it was my fault. The main reason, though, why I don't think there was an adult involved is that my sister has always had a very strong sense of the dramatic. She admits she enjoys an enormous row, even though they leave everyone else feeling awful, and when she is angry she will say the most terrible, hurtful things. There has never really been a line she wouldn't cross and I am pretty sure that, if someone had abused her as a child, she would have denounced them, maybe after they were dead and probably to their nearest and dearest. If our situation was reversed, I don't think she would have waited half a century to say something, and I guess she would have said it in front of my family. I don't know whether that makes her braver or crueller than me, but it is just the way we are.
Thank you again, all of you.