The problem you need to solve: See Sub-Task Priority be updated with Parent Priority .
How Minyaa solves this problem: Minyaa allows you to define a Workflow Function usable in any Transition and able to update Priority by Parent Priority.
By defining this Workflow Function in a Transition of your workflow, you are able to force Sub-Task's Priority to be updated with Priority of Parent Issue.
You can also define a such Workflow Function in the Global Edit Transition (See Issue Validation), and then be sure that the integrity of this rule is respected.
To configure the Inherit Priority Workflow Function, please follow these steps :
<post-function > <function type="class"> <arg name="class.name">com.atlassian.jira.workflow.function.InheritPriorityFunction</arg> </function> </post-function>