It's not something she should expect at all, and she comes over as pretty unreasonable. BUT the IL's shouldn't really have said they would do this if they weren't sure. They do have a right to change their minds but it would be better not to have said it would happen in the first place.
I suppose it depends on whose idea it was - if it was the DIL's and son's idea, and they put pressure on the ILs, who felt bullied into it, then said no, then I', on the ILs' side. If it was the ILs' idea and they suggested it, then changed their minds, that would be hard to take. But still, the letter writer comes across as pretty entitled.