are having twins heredity?
Pretty much, yes. But with only one year's breeding history it can be a bit hit and miss.
Most of us would prefer a first-timer to have a single, and then to have twins every year until her teeth drop out.

I wouldn't discount any of them on the basis of being a single from a shearling.
Chunky maybe is the best conformation? Good in terms of passing that onto her offspring, but could be less good if you use a well-muscled type of terminal sire, in case the lambs are
too chunky and hard to lamb.
Buyers in a fat or store ring pay for buttocks and a 'tight skin' (not a rough fleece.) Maybe length after that.
Personally I don't think you've given enough reason to deselect or select any one. So just keep the one you like best
