Sum Story points in linked Epic
This template displays the sum of the selected story point field from all work items within the epic linked to the current work item.
This includes the item itself, except if it is an epic. Then, all of its child items will be considered for the calculation instead.
Sub-tasks are not considered for the calculation.
If no such items are found, βNoneβ will be displayed.
For the parameter Story points, select the field that represents story points in your instance.
Configuration
To use the Sum Story points in linked Epic template, simply select it from the template grid. Filter by Epic 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
Jira expression:
%{sum(fieldValue({issue.cfNNNNN}, issuesUnderEpic()))}
Replace the ID with the one of your 'Story Points' custom field.
Format
Default
Used parser functions
The links lead to the JWTC documentation because the parser functions are shared functionalities.