It seems the related tables will be shown in the order of how the relationship classes are defined in ArcCatalog. and once the relationship classes are defined, there is no easy way to re-order it on the client side (ArcMap, WebMap)! you have to delete all the relationship classes and re-create the relationship classes in your desired display order for Collector(Webmap) or ArcMap.
I have multiple layers with multiple relationship classes respectively, and I found this is a tedious process to re-order the relationship classes in order to get the right display order in Collector.
I believe there must be a good reason for not to allow users to change the order of he relationship classes in the Geodatabase, but I also believe there should be a better way to change the order of the display on the client side...
It can sort the display order inside the relate table, but the display of the relate tables is still determined by the order the relationship classes are defined in ArcCatalog. Furthermore, I want to sort the display by the importance and relevance of the tables and fields, rather than by the alphabetical orders.
So the ability to manually adjust the order is necessary.