I think it could be to do with the amount of space they have, Hillview. If they are outside, as opposed to in a communal shed, then they can take their lambs well out of butting range of other dams while they settle down. This is probably one of the reasons that ewes about to lamb take themselves away from the main flock, to try to ensure that their lambs imprint on them and not on another ewe. When they are together in limited space each lamb will at some point intrude into the personal space of the other ewe, and be taught manners by that ewe.
When our first lamb arrived a couple of days ago to a first time mum, I was worried that the other ewes, especially the Soays which can be horrid, would bully them. It turned out to be my two Shetlands which were really curious and kept going to see the lamb. The new mum (Heb) gave the Shetlands a really good duffing until they scuttled off slightly offended to another part of the field, and she protected her lamb from them really well (not that they were going to do anything beyond sniffing him). Now he wanders around amongst them and they just accept him. The ancient unbred ewes though really want to get into the lambing paddocks to meet and greet the new arrival.
There's a whole lot of social interactions with sheep, being a flock animal, so there will be dominance already established between the two ewes you have together, and as with chimpanzees for example, the offspring initially inherit the dam's social status. Great fun to observe sheep behaviour.