If I were your sister, I would mentally accept that I failed to redeem my prize within the deadline made clear on what I had as a coupon. That is of course, only if I knew that the photocopy receipt would have been accepted by the store to get a turkey dinner had I gone in the time frame specified (yes, I get that it wasn't set by the store). I would then go to the church and explain what happened in a "hey I think you should know that when this was donated, it was done in this way and here is what happened" sort of mind frame. They should know that this donor went out of their way to set up the donation so that they could benefit if the buyer didn't come through in time so that they can either prevent such donations in the future or warn people that deadlines set on the items are not flexible.
I think this lady wanted to specifically gift a Thanksgiving Dinner, but realized for whatever reason that the gift card wouldn't guarantee that timeframe or that people may not pick up their actual prize. Seeing an opportunity to not be out $50 while still looking charitable, she decided to literally buy a turkey dinner and then take it for herself, if the person didn't show. I think that this was wrong, as the sister paid for it, but the missing of the deadline makes it murky, more so if the sister was aware of the deadline before the purchase (did she?). This isn't the store's fault, though I think their hinky meters should have been going off. It is the fault of the lady and perhaps the church if they did know what she was up to.