I would echo not to pass it off on sis--it's kinda throwing her under the bus if you do.
(although, if sis is OK w/ that--as in, you call her and say "hey, mom is being a twit and wants me to demand you give her the kids' grade info. Are you Ok w/ me just saying "sis never got back to me" verbatim, over and over again?" and she agrees, I'd run with it.)
But "huh, it hasn't been a priority for me yet--hey, how about that bean dip" can work.