I was thinking about what my contract was at the retail places I worked at when at school.
I am pretty sure it was a zero hour contract, but IN GENERAL you knew that you were going to get a minimum of 13hrs/week or whatever on a Tues/Thurs/Sat.
I can't remember the ins & outs exactly but, the zero hour contract meant that they could change your hours/shifts on a weekly basis and to be fair they always tried to give as much notice as possible to achieve this.
I suspect there is nothing in your daughter's contract that says her hours are X hours/week on a Sunday. That's probably just what fits in & suits what everyone else does & what the requirement is.
Presumably if she doesn't want to work it, she doesn't have to, but then won't get paid. Sounds normal for that kind of role.