I answered this first time years ago in Stony Brook Sunday school - 当人种增加前，没有今天复杂的亲属观念。
There was no complication in human relations. Therefore, they were all viewed as Children of God, rather than just brothers and sisters in contrast to other relational ties (i.e. cousins, friends, etc.)
To teach this in Children's Sunday school, simply needs to allude the the teaching of Christ in Matthew 12:50 - that the view of siblings as God's children was as plain as today's view on believers as brothers and sisters in Christ.