Sum completed sibling items’ story points
This template displays the sum of numeric field values in the completed items under the same parent as the current work item.
For a work item to be completed, it needs to be in a status with the status category Done.
Configuration
To use this template, simply select it from the template grid. Filter by KPI to find it faster.
Parameters
To complete the configuration, select values for all parameters.

Expert mode
If you switch to Expert mode you can see the Smart field in the Expression Parser. You can now tweak the expression to create a custom smart field based on this template.
Expression
General expression:
%{sum(fieldValue({issue.cfnnnnn},issuesFromJQL("statusCategory = Done AND 'parent' = "+%{parent.key})))}
Replace "nnnnn" with the ID of the story points field. Keep in mind, that this smart fields should be displayed on a child work item.
Format
Default
Used parser functions
The links lead to the JWTC documentation because the parser functions are shared functionalities.