Practical solution: remove the "sic" and replace it with a "..." to indicate that you've excised a portion of the quote. Or, y'know, don't bother to quote the p

Practical solution: remove the "sic" and replace it with a "..." to indicate that you've excised a portion of the quote. Or, y'know, don't bother to quote the passage with the offending "sic" at all. You could even come up with a way to phrase it so that you're quoting the original rather than quoting the quotation. If you wanted to include it and do it unambiguously you'd need to come up with a convention and state it clearly in the text. "Please note that all instances of "[sic]" which use square brackets are author B's insertion when he quoted author A, while instances of "(sic)" with parentheses are my own insertion in quoting author B." Thus we have "a letter from me [sic] (sic) to you".