Tim, it's too bad that your parents would shun your sister merely over a label, that's just silly if they already know she isn't a JW anymore.
I wouldn't count on a letter working, but it might. I think it's succes is dependant on how much they believe your sister could damage their (the elders) lives individually thru legal action. Showing up with a lawyer might do the trick. BTW, while you might have messing with their minds, there can be no good come from her meeting with them, UNLESS she came with a lawyer to intimidate them (in which case they would not meet).
I'll send you a copy of the letter I sent the elders who came after me recently. It worked, and it didn't work. It put of the df'ing for a little over a month, which gave my family time to enjoy my sister's wedding w/o having to either shun me, or choose me over family friends (which unfortunately, I think I will find out soon that while they won't shun me overtly, I'll be the one disinvited to family get togethers, rather than the family friends who are witnesses).
pm me your e-mail addy.