I'm willing to give the Grandparents a pass for mentioning that they had planned to attend a concert or go to Vegas or wherever Mathis was playing that weekend. I can easily see the OP on the phone with her mother telling her that she didn't know how she was going to make it because of the mix-up on the dates and her mother commisserating by saying she knew, she and her DH made plans for that weekend, too.
Just because they have tickets an event in May doesn't mean they're actually going to go to that event, so I'm not ready to rake the GP's over the coals for something they haven't even done yet. In fact, if they've purchased airline tickets and made hotel reservations, I can understand all the unravelling they're going to have to do now and wanting to vent about it to their daughter.
Perhaps the GP's have changed their plans in order to attend the Communion, but were commisserating over the loss of the concert ticket prices (the dates can't be changed on those). I totally get that.
However, I agree with most of the posters here that the sister should have been very specific on the dates of the Communion. Perhaps if the OP had known the specific date, she wouldn't have volunteered to tear down sets that day and would have been able to see her son in the play AND attend the Communion the next day.
Her sister is just going to have to lump it. If this was such an important event, she should have confirmed with a card, an email or something last fall that told everyone to save the specific date. This is a good lesson in learning that the world (or even the family) doesn't revolve around her DD's milestones. It would be nice if they were there, but (like a wedding) the whole family doesn't have to be there in order for the DD to be confirmed.