Sometimes an udder doesn't have much milk in because the lambs have drunk it all. And sometimes we humans can't get milk out of the udder, but it's there - she just won't drop it for us.
However, if the udder seems small, the teats floppy rather than full, and the lambs hungry, then she doesn't have enough for them.
If you have any grass you can put her on, that will help her milk supply more than anything else you can do. And a high protein feed - but I'm not sure how much you should give a wee Boreray, even one with twins.
The jag that helps them drop the milk is oxytocin, but generally the hormones at birth and the feeling of the lambs suckling are all the stimulus a ewe needs. Some first timers do take 24 hours or so to really get the milk supply going, in which case yes, give colostrum to the lambs as they must have this with the first 6 hours really.