if they're not redemptionists themselves, they're probably sinners anyways
This..

or they are attacking to press gang them into service knowing they are easier to convert.
"whats that? you dont want to leave your family to join the church..."
*sound of purging flames*
"there now you dont have a family"
If there was a way to have your parent civ never send a caravan I would do that, and make another caravan only civ. Would be a good workaround for some current problems.
I did have an idea about RAWs that randomise before each worldgen (or on command with a batchfile) but I dont have any idea how to code textparsers/modifiers. There have been other attempts to make this on B12 in the past but none ever really worked.
IF this were possible we could have a random house gang come to trade with you, and another that comes to attack you, cycling them with each worldgen. CRAZY ideas!