Within a competency family you can create "rules" between two of the children. You cannot create a "rule" between children of separate families.
Depending on how to built your competency framework, the rules may or may not be usable. If you have a single parent that will have all the same scales, you may want to see where you would want to link children. If the student is awarded competency A they will also achieve competency B.
I have many parents that use different scales - so I cannot use rules.