So maybe I’m a late to this party, but the maidens that treat Rand as a son rather than a brother, could they be one that had given birth and gave their child to a wise one? It would make sense that they would treat Rand as a son. Or is it purely preferential?

While that could definitely play a role, the book makes clear that age at least doesn't determine how they treat him. I think it explicitly uses examples of young who treat him like a son and old who treat him like a brother. 

 

But yes it definitely makes sense that those that gave a baby up for adoption would be more inclined to see him a representative of all those children given up. 

I always thought the Maidens that treated him as a sin were those that had given up a child. There also seemed 2 different levels of treatment as a brother. More intense (like Sulin) I thought were women whose sister or or family member had given up a child, whereas the lesser intense brotherly treatment were Maidens with no close associations