OK gyps - knowing that I probably couldn't do this...i would suggest tell him you are going to call sis and deal directly with her about this, then call sis and tell her you didn't, why did she think you did, etc. It's best not to get into 3 ways (well.....)
But I probably wouldn't do that cause i am a chicken. Just one of many options. Also - when karmas clash I try to meditate and repeat "may we walk skillfully through our karma" and ask that WE (all parties) be given intuitve knowledge of how to do this from the buddha nature.
Love you!
AHHHHHRG