My girls all get on realy well but i always keep them apart when one of them has a litter.I have found that even the normally very placid girls can get very snappy and over protective of their pups when one of the other dogs comes near.
To overcome this i just use a baby gate to the room where mum and pups are and i cover it with a blanket so the other dogs dont nose in and upset mum.
My girls are only possesive when they are still feeding and as soon as the pups start to be weaned they then dont mind the other dogs taking a look.
I would play it by ear as your girl may be fine but its certainly not worth upsetting mum ,as if mum gets stressed and upset then she may not feed her pups and some mums have also been known to hurt their pups.