What is Ontario law about working on sundays?

In Ontario, Canada, I believe there is a law that restricts employers to require workers from working on week ends for religious purposes. I'm not sure if this applies to ALL jobs and what exceptions are there, ex. part-time, company based, etc, but my sister who is a full time banquet manager has to work most sundays and it interferes with church. If someone knows about this or where to look, it would be much appreciated! Thanks.