I don't think for a minute the Muslim parents are home eating popcorn and are refusing to help just to be difficult (very judgemental though), we don't know what their schedule is like. I'm also sure by having a whole school board issue over it they feel really welcome right?
When I spent tons of hours volunteering at my daughter's elementary school, very little of my time actually was interacting with her. I would be happy to supervise those children and I'm as religious as my diet pepsi bottle.
I hope your sister sees that her child as well as MANY others need additional supervision and has altered her schedule to do the right thing.
For dinner...room service at the Marriott.