Non corp jjob, unpaid leave?

My boss told me I can't sit at all at my job, I'm a snack bar attendent so I basically have to be on my feet but when its slow I would like to sit seeing as I'm 20 weeks preg and can only see standin gettin harder. He knows I'm preg and my manger said it was fine but the boss said no... Or I'd have to go on leave. It's a family owned business not my family bit there for it is non corp and my leave would be unpaid. I'm already takin 6 weeks after the baby and I don't want to have to start early cuz of sitting bit a lot of my friends said its not rite and I was wondering. Do the have the rite to tell a preg women she can't sit or be forced to go on unpaid leave?